Student-Friendly Dictionary of Chemistry: 250+ Terms

Chemistry is not just about formulas and reactions; it is also a language filled with thousands of scientific terms. Many students find chemistry difficult not because of calculations, but because they do not fully understand the meaning of important words like mole, oxidation, catalyst, or stoichiometry.

A Dictionary of Chemistry acts as a bridge between complex scientific language and simple understanding. It helps students quickly revise important terms, strengthen their concepts, and improve performance in exams.

In this article, we present a student-friendly dictionary of chemistry terms with clear definitions, easy explanations, and examples wherever required.

Why Every Chemistry Student Needs Dictionary of Chemistry

• Improves conceptual understanding
• Helps in faster revision
• Increases exam accuracy
• Builds scientific vocabulary
• Reduces fear of chemistry

A

Absolute Zero: The theoretical temperature (0 K or -273.15°C) at which all molecular motion ceases.

Absorption Spectrum: A spectrum produced when atoms absorb specific wavelengths of light, appearing as dark lines on a bright background.

Accuracy: The measure of how close a value is to the true or accepted value.

Acid Dissociation Constant (K_a): A quantitative measure of the strength of an acid in solution.

Acidic Oxide: A non-metal oxide that reacts with water to form an acidic solution, e.g., SO₂.

Actinoids: The series of 14 inner transition elements from Thorium (Th) to Lawrencium (Lr).

Activated Complex: An intermediate structure formed during the collision of reactant molecules with maximum energy.

Activation Energy (E_a): The minimum energy required to start a chemical reaction.

Addition Reaction: A reaction where two or more molecules combine to form a larger one (typical of alkenes/alkynes).

Adsorption: The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid.

Alcohol: An organic compound containing a hydroxyl (-OH) group bonded to a saturated carbon atom.

Aldehyde: An organic compound containing a carbonyl group with at least one hydrogen atom attached (-CHO).

Aldol Condensation: A reaction where aldehydes or ketones containing at least one alpha-hydrogen react with dilute alkali to form beta-hydroxy aldehydes (Aldols) or beta-hydroxy ketones (Ketols), which upon heating undergo dehydration to form αalpha, βbeta-unsaturated carbonyl compound.

Aliphatic: Organic compounds where carbon atoms form open chains, not aromatic rings.

Alkali Metals: Elements of Group 1 (Li, Na, K, etc.) which form strong bases with water.

Alkaline Earth Metals: Elements of Group 2 (Be, Mg, Ca, etc.).

Allotropy: The existence of a chemical element in two or more forms in the same physical state (e.g., Diamond and Graphite).

Alpha Particle: A helium nucleus (He²⁺) emitted during radioactive decay.

Amine: Derivatives of ammonia where one or more hydrogen atoms are replaced by alkyl/aryl groups.

Amino Acid: The building blocks of proteins containing both an amino (-NH₂) and carboxyl (-COOH) group.

Amorphous Solids: These do not have definite geometrical structure because the constituent particles are not arranged in an orderly manner but in a random manner.

Ampere: It is defined as the current that, when flowing through each of two long parallel wires separated by 1 meter of free space, results in a force between the wires of 2 x 10^-7 newton per meter of length.

Amphoteric Oxide: An oxide that can act as either an acid or a base (e.g., Al₂O₃).

Amplitude: It is the height of the crest or depth of trough of a wave. It determines the intensity or brightness of light.

Anion: A negatively charged ion.

Anode: The electrode where oxidation (loss of electrons) takes place. In galvanic cells, it is negative; in electrolytic cells, it is positive.

Antibonding Molecular Orbital: A molecular orbital containing electrons that destabilize the molecule (higher energy than atomic orbitals).

Aqueous Solution: A solution in which the solvent is water.

Aromaticity: A property of cyclic, planar molecules with delocalized pi-electrons (4n+2 rule) that gives extra stability.

Arrhenius Equation: k = A e^-E_a/RT, it relates rate constant, temperature, and activation energy.

Atom: It is the smallest indivisible particle of an element that can take part in a chemical reaction.

Atomic Mass Unit (amu/u): One-twelfth of the mass of a carbon-12 atom.

Atomic Number: The number of protons present in an atom. Represented by Z.

Atomic Orbitals: The region in space around the nucleus in which there is high probability of finding the electron is called orbital. The orbitals are of four types:

Orbitals	Shapes	Maximum number of orbitals in different subshell	Maximum number of electrons
s	Spherical	One	2
p	Dumb-bell	Three	6
d	Double dumb-bell	Five	10
f	Complex	Seven	14

Atomic Radius: The distance from the nucleus to the outermost electron shell.

Atomic Volume: The volume occupied by one mole atoms of an element.

Aufbau Principle: A rule stating that in the ground state of an atom, electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels. The order of increasing energies is: 1s < 2s < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p.

Avogadro’s Constant (N_A): 6.022 x 10²³ particles per mole.

Avogadro’s Law: Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

Azeotrope: A binary mixture of liquids that has the same composition in its liquid and vapor phase and boils at a constant temperature.

Azimuthal Quantum Number: It denotes the sub-level to which the electron belongs, its shape and energy associated with angular momentum of the electron. Represented by ‘l’. The energies of the various sub-shell are in the order of s < p < d < f. The maximum number of electrons which can be held by these sub-levels is given by 2(2l+1)

B

Balmer Series: A series of spectral lines of the hydrogen atom in the visible region.

Base: A substance that accepts protons (Bronsted) or donates electron pairs (Lewis).

Basic Buffer: A buffer solution consisting of a weak base and its salt with a strong acid.

Battery: An arrangement of electrochemical cells used as an energy source.

Bauxite: The primary ore of aluminum (Al₂O₃.2H₂O).

Benzene: The simplest aromatic hydrocarbon (C₆H₆).

Benzenoid: Aromatic compounds containing at least one benzene ring.

Beta Elimination: A reaction where a functional group and a hydrogen atom are removed from adjacent carbons to form a double bond.

Bidentate Ligand: A ligand that bonds to a central metal atom through two donor atoms (e.g., Ethylenediamine).

Bimolecular Reaction: A reaction where two reacting species collide in the rate-determining step.

Biochemistry: The study of chemical processes within and relating to living organisms.

Biodegradable Polymer: Polymers that can be decomposed by bacteria (e.g., PHBV).

Biomolecule: Organic molecules essential for life (Carbohydrates, Proteins, Nucleic Acids).

Body-Centered Cubic (BCC): A unit cell structure with lattice points at corners and one in the center of the body.

Boiling Point: The temperature at which the vapor pressure of a liquid equals the external pressure.

Bond Angle: The angle between two orbitals containing bonding electron pairs around the central atom.

Bond Dissociation Enthalpy: The energy needed to break a specific bond in one mole of gaseous molecules.

Bond Length: The equilibrium distance between the nuclei of two bonded atoms.

Bond Order: Half the difference between the number of electrons in bonding and antibonding orbitals. The number of chemical bonds between a pair of atoms. For a molecule to be stable, the bond order must be positive.

Bond Polarity: The distribution of electrical charge over the atoms joined by the bond.

Born-Haber Cycle: A thermochemical cycle used to calculate lattice energy.

Boyle’s Law: At constant temperature, the volume of a given mass of a gas is inversely proportional to its pressure. P₁V₁ = P₂V₂

Bronsted-Lowry Acid: A proton (H⁺) donor.

Buffer Capacity: The efficiency of a buffer to resist pH change.

Buffer Solution: A solution that resists changes in pH upon addition of small amounts of acid or base.

C

Calcination: Heating an ore in the absence of air to remove volatile impurities (usually for carbonates).

Calorimetry: The experimental technique of measuring the heat change in a chemical reaction.

Candela: It is defined as the luminous intensity of 1/600,000 of a square meter of a black body at the temperature of frezzing platinum (2045K).

Cannizzaro Reaction: A reaction of aldehydes without alpha-hydrogens (like HCHO) with concentrated alkali, undergoing self-oxidation and reduction (disproportionation).

Carbohydrate: Polyhydroxy aldehydes or ketones (e.g., Glucose, Starch).

Carbocation: An organic ion with a positive charge on a carbon atom.

Carbonyl Group: A functional group composed of a carbon atom double-bonded to an oxygen atom (C=O).

Carboxylic Acid: An organic compound containing the -COOH group.

Catalyst: A substance that increases the rate of reaction without being consumed.

Catenation: The property of self-linking of atoms of an element to form chains and rings (Carbon excels at this).

Cathode: The electrode where reduction (gain of electrons) occurs.

Cation: A positively charged ion.

Cell Potential (E_cell): The difference between the electrode potentials of the cathode and anode.

Charles’s Law: At constant pressure, the volume of a given mass of a gas is directly proportional to absolute temperature. V₁T₂ = V₂T₁

Chelate: A complex formed by a polydentate ligand forming a ring structure with the metal.

Chelate Effect: The increased stability of a coordination compound due to the formation of ring structures by polydentate ligands.

Chemical Equilibrium: The state where the rates of the forward and reverse reactions are equal.

Chemical Kinetics: The branch of chemistry studying reaction rates and mechanisms.

Chirality: The geometric property of a molecule being non-superimposable on its mirror image.

Chromatography: A technique for separating components of a mixture based on differential adsorption.

Cis-Isomer: A geometric isomer where identical groups are on the same side of the double bond.

Coagulation: The process of settling of colloidal particles.

Colligative Properties: Properties depending on the number of solute particles, not on the nature of solute particles (e.g., Osmotic Pressure).

Collision Theory: Reaction rates depend on the frequency and energy of collisions between molecules.

Complex Ion: A central metal ion surrounded by ligands.

Concentration: The amount of solute present in a given quantity of solvent or solution.

Condensation Polymerization: Polymerization where monomers join with the loss of a small molecule like water (e.g., Nylon).

Conductivity: The conductance of a solution of 1 cm length and 1 square cm cross-section.

Conjugate Acid-Base Pair: An acid and a base that differ by only one proton (H⁺).

Coordination Number: The number of ligand donor atoms bonded to a metal or total number of coordinate bonds formed by a metal with donor atom of ligands.

Corrosion: The oxidative deterioration of a metal (e.g., Rusting).

Covalent Bond: A bond formed by the sharing of electron pairs.

Covalent Radius: Half of the internuclear distance between two atoms of the element held by a single covalent bond.

Critical Pressure: The pressure that must be applied on 1 mole of a gas to liquefy it at its critical temperature.

Critical Temperature: The temperature of a gas above which it can’t be liquefied whatever high pressure applied is critical temperature.

Critical Volume: It is the volume occupied by 1 mole of a gas at its critical temperature and critical pressure.

Crystal: A crystalline solid consists of a large number of small units, called crystals.

Crystal Field Theory: A model describing the breaking of degeneracies of electron orbital states (d-orbitals) due to ligands.

Crystalline solids: A crystalline solid is characterised by a definite crystal structure, a definite melting point and a definite heat of fusion.

D

Dalton’s Law: The total pressure of a gas mixture is the sum of partial pressures of individual gases.

Daniell Cell: A galvanic cell using Zinc and Copper electrodes.

De-Broglie Equation: Inverse relation of the wavelength of a particle to its momentum (λ = h/mv).

Decantation: Separating a liquid from a solid sediment by pouring it off.

Decomposition Reaction: A reaction where a compound breaks down into simpler substances.

Degree of Ionization: The fraction of total molecules that ionize in solution.

Dehydration: Removal of water from a compound (e.g., Ethanol to Ethene).

Delocalization: Electrons that are spread over several atoms rather than localized between two.

Denaturation: The disruption of the natural structure of a protein or nucleic acid.

Density: Mass per unit volume.

Depression in Freezing Point: A colligative property where adding a solute lowers the freezing point.

Desorption: The removal of an adsorbed substance from a surface.

Dextrorotatory: Rotates plane-polarized light to the right (+).

Diagonal Relationship: Similarities between elements in diagonal positions of the Periodic Table (e.g., Li and Mg).

Diamagnetic Substances: These substances are weakly repelled by the external magnetic field (no unpaired electrons), and the property is called as diamagnetism.

Diastereomers: Stereoisomers that are not mirror images of one another.

Diazotization: The reaction of primary aromatic amines with nitrous acid to form diazonium salts.

Diffusion: The net movement of particles from high to low concentration.

Dilution: Reducing the concentration of a solute by adding more solvent.

Dipole Moment: A measure of the polarity of a molecule.

Disaccharide: A carbohydrate formed by two monosaccharides (e.g., Sucrose).

Displacement Reaction: An element replaces another element in a compound.

Disproportionation: A redox reaction where an element is simultaneously oxidized and reduced.

Dissociation: Separation of ions when an ionic compound dissolves.

Double Bond: A covalent bond sharing two pairs of electrons.

Double Salt: A crystalline compound of two different salts that dissociates completely into ions (e.g., Mohr’s Salt).

Dynamic Equilibrium: A state of balance between continuing processes (forward and reverse rates equal).

Read more: Unique Guide to Organic Conversions (25 Cheat Sheet included)

E

Effective Nuclear Charge (Z_eff): The net positive charge experienced by an electron in a polyelectronic atom.

Effusion: If a gas is allowed to escape from its container through a small hole into vacuum.

Electrochemical Cell: A device converting chemical energy to electrical energy (or vice versa).

Electrochemical Series: A list of elements ordered by their standard electrode potentials.

Electrode Potential: The potential difference between the electrode and the electrolyte.

Electrolysis: The decomposition of a substance by passing an electric current.

Electrolyte: A substance that conducts electricity when dissolved or molten.

Electromagnetic radiations: These are the waves which can be produced by moving a charged body in a magnetic field or a magnet in an electric field. Ordinary light rays, X-rays, gamma-rays, etc. are such radiations. They have wave characteristics and do not require any medium for propagation.

Electromagnetic spectrum: The arrangement of the various types of electromagnetic radiations in order of their increasing or decreasing frequencies or wavelengths.

Electron: The cathode ray particles having negative charge and a specific charge to mass ratio remains constant irrespective of nature of gas in discharge tube are known as electrons. Charge of electron is -1.6 x10^-19 C and mass of electron is 9.1 x 10^-31 Kg.

Electron Affinity (Electron Gain Enthalpy): The energy released when an electron is added to a neutral gaseous atom.

Electronegativity: The tendency of an atom to attract a shared pair of electrons towards itself.

Electronic Configuration: The arrangement of electrons in the orbitals of an atom.

Electrophile: An electron-deficient species that attacks electron-rich areas.

Electroplating: Coating a metal object with a thin layer of another metal using electrolysis.

Elementary Reaction: A reaction that occurs in a single step.

Elimination Reaction: Removal of atoms/groups from a molecule to form a double bond.

Empirical Formula: The simplest whole-number ratio of atoms in a compound.

Emulsion: A colloid of two or more immiscible liquids (e.g., Milk).

Endothermic: A process that absorbs heat (ΔH > 0).

Endpoint: The point in a titration where the indicator changes color.

Enthalpy (H): The total heat content of a system.

Entropy (S): A measure of disorder or randomness.

Enzyme: A biological catalyst (protein).

Equilibrium Constant (K_c): The ratio of product concentrations to reactant concentrations at equilibrium.

Equivalence Point: The point in a titration where chemically equivalent amounts of reactants have mixed.

Ester: An organic compound derived from an acid and an alcohol (Sweet smelling).

Esterification: The reaction between a carboxylic acid and an alcohol to form an ester.

Ether: An organic compound with an oxygen atom connected to two alkyl/aryl groups (R-O-R).

Evaporation: Vaporization of a liquid at the surface below its boiling point.

Excited State: A state where an atom/molecule has more energy than the ground state.

Exothermic: A process that releases heat (ΔH < 0).

Extensive Property: A property that depends on the amount of matter (e.g., Mass, Volume).

F

Face-Centered Cubic (FCC): A unit cell structure where atoms are located at each corner and the center of each face.

Faces: Crystal possessed a definite geometric shape bound by plane surfaces called faces.

Faraday Constant: The charge of one mole of electrons (96485 C/mol).

Faraday’s First Law of Electrolysis: The mass of a substance deposited at an electrode is directly proportional to the charge passed (m = ZIt).

Faraday’s Second Law: When the same quantity of electricity is passed through different electrolytes, the masses deposited are proportional to their equivalent masses.

Fatty Acid: A long-chain carboxylic acid found in fats and oils.

Ferromagnetic Substances: These substances are strongly attracted by the external magnetic field. These are permanantly magnetised, and the property is called as ferromagnetism.

Fibrous Protein: Proteins consisting of linear polypeptide chains that form fibers (e.g., Keratin, Myosin).

Filtration: A mechanical separation method used to remove solid particles from a fluid.

First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed (ΔU = q + w).

First Order Reaction: A reaction where the rate depends linearly on the concentration of only one reactant.

Fission (Nuclear): The splitting of a heavy nucleus into lighter nuclei with the release of energy.

Flame Test: An analytical procedure used to detect the presence of certain metal ions based on the color of the flame produced.

Fluorescence: The emission of light by a substance that has absorbed light or other electromagnetic radiation.

Formal Charge: The difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure.

Formation Constant (K_f): The equilibrium constant for the formation of a complex ion from its central metal ion and ligands.

Formula Mass: The sum of the atomic masses of ions in a formula unit of an ionic compound.

Fractional Distillation: A method to separate liquid mixtures with close boiling points using a fractionating column.

Free Radical: An uncharged molecule (typically highly reactive) having an unpaired valence electron.

Freezing Point: The temperature at which the solid and liquid phases of a substance are in equilibrium.

Frequency: The number of times a wave passes through a given point in one second.

Friedel-Crafts Acylation: Introduction of an acyl group (-COR) into an aromatic ring using an acyl halide and anhydrous AlCl₃.

Friedel-Crafts Alkylation: Introduction of an alkyl group (-R) into an aromatic ring using an alkyl halide and anhydrous AlCl₃.

Fuel Cell: A cell that converts the chemical energy of a fuel (like H₂) directly into electricity.

Functional Group: An atom or group of atoms within a molecule that shows a characteristic set of chemical reactions (e.g., -OH, -COOH).

Fundamental particles: Electrons, protons and neutrons are the most important fundamental particles of all atoms (expect hydrogen).

G

Gabriel Phthalimide Synthesis: A chemical reaction used to prepare primary amines from phthalimide.

Galvanic Cell (Voltaic Cell): An electrochemical cell that derives electrical energy from spontaneous redox reactions.

Gamma Ray: High-energy electromagnetic radiation emitted by radioactive decay.

Gangue: The commercially valueless material (sand, rock, etc.) in which ore is found.

Gas Constant (R): The constant in the ideal gas law (8.314 J/K mol).

Gaseous State: When the forces of attraction between the particles of a matter is minimum, the particles exists in a state called gaseous.

Gas Equation: The equation combines Boy;e’s law and Charles’s law. For ‘n’ moles of a gas, the gas equation is PV = nRT.

Gay-Lussac’s Law: According to this law, pressure of a given mass of gas is directly proportional to absolute temperature at constant volume. P₁T₂ = P₂T₁

Gel: A colloid in which the dispersed phase is liquid and the dispersion medium is solid (e.g., Jelly, Cheese).

Geometric Isomerism: Isomerism resulting from restricted rotation, creating cis and trans forms.

Gibbs Energy (ΔG): A thermodynamic potential that measures the maximum reversible work a system can perform. Negative ΔG implies spontaneity.

Gibbs-Helmholtz Equation: ΔG = ΔH – TΔS.

Globular Protein: Spherical proteins that are soluble in water (e.g., Insulin, Hemoglobin).

Glucose: A simple sugar (C₆H₁₂O₆) and a key energy source for living organisms.

Glycosidic Linkage: The bond formed between two monosaccharides through an oxygen atom.

Graham’s Law of Diffusion: The rate of diffusion of a gas is inversely proportional to the square root of its molar mass.

Gravimetric Analysis: Analytical method to determine the amount of an analyte based on the mass of a solid.

Grignard Reagent: An organometallic reagent (RMgX) used to form carbon-carbon bonds.

Ground State: The lowest energy state of an atom or molecule.

Group: A vertical column in the periodic table; elements in a group have similar valence electron configurations.

H

Haber Process: The industrial process for manufacturing ammonia (NH₃) from nitrogen and hydrogen.

Half-Cell: One part of an electrochemical cell (either the oxidation or reduction side).

Half-Life (t_1/2): The time required for a quantity (reactant concentration) to fall to half of its value.

Haloalkane: An alkane where one or more hydrogens are replaced by halogen atoms (Alkyl Halide).

Haloarene: An aromatic compound where a halogen is bonded directly to the benzene ring (Aryl Halide).

Halogen: Group 17 elements (F, Cl, Br, I, At).

Hard Water: Water containing high concentrations of calcium and magnesium ions.

Heat Capacity: The amount of heat required to raise the temperature of a substance by 1 ^οC.

Heat of Combustion: The heat energy released when a substance burns completely in oxygen.

Heisenberg Uncertainty Principle: It is impossible to determine both the position and momentum of a particle with absolute precision.

Hell-Volhard-Zelinsky (HVZ) Reaction: Halogenation of carboxylic acids at the alpha-carbon.

Hemiacetal: A compound formed by the addition of one alcohol molecule to an aldehyde.

Henderson-Hasselbalch Equation: Used to calculate the pH of a buffer solution: pH = pK_a + log {[Salt]/[Acid]}.

Henry’s Law: The solubility of a gas in a liquid is directly proportional to the partial pressure of that gas.

Hess’s Law: The total enthalpy change of a reaction is independent of the pathway taken.

Heterogeneous Catalyst: A catalyst in a different phase (usually solid) than the reactants (usually gas/liquid).

Heterolytic Cleavage: Bond breaking where one atom takes both electrons, forming ions.

Hexagonal Close Packing (HCP): An efficient way of packing spheres (atoms) in a crystal lattice (coordination number 12).

Hoffmann Bromamide Degradation: Reaction of a primary amide with Br₂/NaOH to form a primary amine with one less carbon.

Homogeneous Catalyst: A catalyst in the same phase as the reactants.

Homologous Series: A family of organic compounds with the same functional group and similar chemical properties.

Hund’s Rule: According to this rule, electron pairing in any orbital (s, p, d or f) cannot take place until each orbital of the same sub-level contains 1 electron.

Hybridization: The mixing of atomic orbitals to form new equivalent hybrid orbitals (sp, sp², sp³).

Hydration Enthalpy: The energy released when ions are surrounded by water molecules.

Hydride: A binary compound of hydrogen with an element.

Hydrogen Bond: A strong dipole-dipole attraction between a hydrogen atom bonded to N, O, or F and another electronegative atom.

Hydrolysis: Breaking a chemical bond using water.

Hydronium Ion: The aqueous cation H₃O⁺.

Hydrophobic/Hydrophilic: Water-fearing (non-polar) / Water-loving (polar).

Hyperconjugation: Delocalization of sigma-electrons (C-H bond) into an adjacent empty p-orbital (“No-bond resonance”).

I

Ideal Gas: A hypothetical gas that perfectly obeys the gas laws (PV=nRT) at all conditions.

Ideal Solution: A solution that follows Raoult’s law over the entire range of concentrations (ΔH_mix=0, ΔV_mix=0).

Indicator: A substance that changes color at a specific pH range (e.g., Phenolphthalein).

Inductive Effect (+I / -I): The transmission of unequal sharing of the bonding electron pair through a chain of atoms in a molecule.

Inert Pair Effect: The reluctance of the s-electrons of the valence shell to take part in bonding (common in heavier p-block elements like Pb, Bi).

Inhibitor: A substance that decreases the rate of a chemical reaction.

Initiation Step: The first step in a radical chain reaction where radicals are formed.

Inner Transition Elements: Lanthanoids and Actinoids (f-block elements).

Instantaneous Rate: The rate of reaction at a specific instant in time.

Interhalogen Compounds: Compounds formed between two different halogen elements (e.g., ICl, BrF₃).

Intermediate: A species formed during a reaction that reacts further to give the product; not present in the final product.

Intermolecular Forces: Forces of attraction/repulsion between molecules (Van der Waals, H-bonding).

Internal Energy (U): The sum of kinetic and potential energies of the particles in a system.

Interstitial Compound: A compound formed when small atoms (H, C, N) are trapped inside the crystal lattice of metals.

Intramolecular: Occurring within the same molecule (e.g., Intramolecular Hydrogen Bonding).

Invert Sugar: An equimolar mixture of glucose and fructose obtained by the hydrolysis of sucrose.

Ionic Bond: Electrostatic attraction between oppositely charged ions.

Ionic Product of Water (K_w): The product of [H⁺] and [OH^–]$ in water (10^-14 at 25°C).

Ionic Radius: The effective distance from the nucleus of the ion to the point where it influences the electron cloud.

Ionization Enthalpy: Energy required to remove the most loosely bound electron from an isolated gaseous atom.

Isobars: Atoms with the same mass number but different atomic numbers.

Isoelectronic: Species having the same number of electrons (e.g., Na⁺ and Ne).

Isolated System: A system that exchanges neither energy nor matter with its surroundings.

Isomerism: The phenomenon where compounds have the same molecular formula but different structures or spatial arrangements.

Isotopes: Atoms of the same element with different numbers of neutrons.

Isotropic: Having physical properties that are identical in all directions.

IUPAC Name: The systematic method of naming chemical compounds.

J

Joule (J): The SI unit of energy, work, or heat.

Joule-Thomson Effect: The temperature change of a real gas or liquid when it is forced through a valve or porous plug while kept insulated (Thermodynamics).

K

Kelvin: It is defined as the fraction 1/273.15 of the temperature of the absolute zero, triple point of water.

Ketone: An organic compound containing a carbonyl group (C=O) bonded to two carbon groups.

Kilogram: It is defined as the mass of a cylinder of platinum-iridium alloy kept by the international bureau of weights and measures at Paris.

Kinetics: See Chemical Kinetics.

Kinetic Energy: Energy possessed by an object due to its motion (mv²/2).

Kinetic Molecular Theory: A theory explaining the behavior of gases based on the motion of particles.

Kohlrausch’s Law: The limiting molar conductivity of an electrolyte is the sum of the conductivities of its ions.

Kolbe’s Electrolysis: An electrochemical method to prepare alkanes from carboxylic acid salts.

Kolbe’s Reaction: Reaction of phenol with CO₂ and alkali to form Salicylic Acid.

Kraft Temperature: The temperature above which the formation of micelles takes place.

L

Lanthanoid Contraction: The steady decrease in atomic/ionic radii of lanthanoids due to poor shielding by f-electrons.

Lanthanoids: The series of 14 elements from Cerium (Ce) to Lutetium (Lu).

Lattice Enthalpy: The energy change when one mole of an ionic solid is formed from its gaseous ions.

Lattice Points: The points representing the atoms, molecules or ions in a unit cell are known as lattice points.

Law of Conservation of Mass: Mass is neither created nor destroyed in a chemical reaction.

Law of Mass Action: The rate of a chemical reaction is proportional to the product of the active masses of the reactants.

Le Chatelier’s Principle: If a dynamic equilibrium is disturbed, the position of equilibrium moves to counteract the change.

Lewis Acid: An electron-pair acceptor.

Lewis Base: An electron-pair donor.

Lewis Dot Structure: A diagram showing the bonding between atoms and lone pairs of electrons.

Ligand: An ion or molecule that binds to a central metal atom to form a coordination complex.

Limiting Molar Conductivity: Molar conductivity at infinite dilution.

Limiting Reagent: The reactant that is completely consumed and limits the amount of product formed.

Line Spectrum: A spectrum consisting of discrete lines (characteristic of atoms).

Lipids: A diverse group of organic compounds including fats, oils, and waxes.

Liquid State: A state of matter with definite volume but no definite shape.

London Dispersion Forces: Weak intermolecular forces arising from temporary dipoles.

Lone Pair: A pair of valence electrons that are not shared with another atom.

Lucas Test: A test using ZnCl₂/HCl to distinguish between primary, secondary, and tertiary alcohols.

Lyophilic Colloid: “Solvent-loving” colloid (stable).

Lyophobic Colloid: “Solvent-hating” colloid (unstable).

M

Magnetic Moment: A measure of the magnetic strength of a substance/ion, related to unpaired electrons (μ = sqrt{n(n+2)} BM).

Magnetic Quantum Number: This number determines the preferred orientations of orbitals in space. Represented ny ‘m’. For each value of l, there are 2l+1 values of m.

Malleability: The ability of a material to be hammered or pressed into shape without breaking.

Markovnikov’s Rule: In the addition of HX to an unsymmetrical alkene, the H attaches to the carbon with more hydrogens.

Mass Number: The sum of number of protons and neutrons. Represented by A. It is always a whole number.

Mass Percent: The mass of a component divided by the total mass of the mixture and then multiplied by 100.

Mass Spectrometry: An analytical technique that measures the mass-to-charge ratio of ions.

Matter: Anything that has mass and takes up space.

Mechanism: The step-by-step sequence of elementary reactions by which overall chemical change occurs.

Melting Point: The temperature at which a solid becomes a liquid.

Meso Compound: A molecule with chiral centers that is achiral (superimposable on its mirror image) due to internal symmetry.

Mesons: Pi-mesons and mu-mesons may be either positively or negatively charged. Their mass is intermediate between that of electron and proton.

Metallic Bond: The attraction between positive metal ions and delocalized “sea” of electrons.

Metallic Radius: Half of the internuclear distance between two nearest atoms in the metallic lattice.

Metallic Character: The tendency of an element to lose electrons and form positive ions.

Metalloid: An element with properties intermediate between metals and non-metals (e.g., Si, Ge).

Metamerism: Isomerism arising from unequal distribution of alkyl groups on either side of a functional group.

Meter: It is defined as the length equal to 1,650,763.73 wavelengths in vacuum of the orange red of the spectrum of Krypton-86.

Micelle: An aggregate of surfactant molecules (like soap) in a colloidal solution.

Molality (m): Moles of solute per kilogram of solvent.

Molar Conductivity: The conducting power of all the ions produced by one mole of electrolyte.

Molar Heat Capacity: It is defined as the quantity of heat required to raise the temperature of 1 mole of substance through 1 °C.
Molar Heat Capacity = Specific heat x Molecular weight

Molar Heat Capacity at constant pressure (C_p): It is defined as the quantity of heat required to raise the temperature of 1 mole of substance through 1 °C under constant pressure.

Molar Heat Capacity at constant volume (C_v): It is defined as the quantity of heat required to raise the temperature of 1 mole of substance through 1 °C under constant volume. C_p – C_v = R

Molar Mass: The mass of one mole of a substance (g/mol).

Molarity (M): Moles of solute per liter of solution.

Molar Volume: 22.4 liters of every gas at NTP weigh equal to its 1 gram mole (molecular weight in grams).

Mole: It is defined as the amount of a substance that contains as many entities as there are atoms in exactly 0.012 kilogram of carbon-12. It is the SI unit for the amount of substance 6.022 x 10²³ particles).

Mole Fraction: The ratio of moles of one component to the total moles of the mixture.

Molecular Formula: The actual number of atoms of each element in a molecule.

Molecular Orbital Theory (MOT): A method for determining molecular structure in which electrons are not assigned to individual bonds but are treated as moving under the influence of the nuclei in the whole molecule.

Molecularity: The number of molecules reacting in an elementary step.

Monodentate Ligand: A ligand that has only one donor atom.

Monomer: A small molecule that reacts with others to form a polymer.

Monosaccharide: The simplest form of sugar (e.g., Glucose, Fructose).

Mutarotation: The change in the optical rotation of a solution (e.g., glucose) over time until it reaches equilibrium.

N

Nanochemistry: The study of the synthesis and properties of materials at the nanoscale (1–100 nm).

Natural Rubber: A polymer of isoprene (2-methyl-1,3-butadiene).

Nernst Equation: Relates the reduction potential of a half-cell to the standard electrode potential, temperature, and concentration {E=E^o – [RT/nF] ln (Q)}.

Neutralization: The reaction between an acid and a base to form salt and water.

Neutron: A neutral particle having nearly the same mass as that of the proton, discovered by Sir James Chadwick is known as neutron.

Nitration: The introduction of a nitro group (-NO₂) into an organic compound (usually using HNO₃ + H₂SO₄).

Noble Gases: Group 18 elements (He, Ne, Ar, etc.) known for their lack of reactivity.

Nodal Plane (Node): A region in an orbital where the probability of finding an electron is zero.

Non-Ideal Solution: A solution that deviates from Raoult’s Law (shows positive or negative deviation).

Non-Polar Covalent Bond: A bond where electrons are shared equally between atoms.

Non-Spontaneous Process: A process that requires continuous input of energy to occur (ΔG > 0).

Normal Boiling Point: The boiling point of a liquid at 1 atm pressure.

Normality (N): The number of gram-equivalents of solute per liter of solution.

Nuclear Charge: The total positive charge of the nucleus (equal to atomic number Z).

Nucleic Acid: Biopolymers (DNA/RNA) essential for all known forms of life.

Nucleophile: An electron-rich species that attacks a positive center (nucleus loving).

Nucleophilic Addition: A reaction where a nucleophile attacks a carbonyl carbon (common in aldehydes/ketones).

Nucleophilic Substitution: A reaction where a nucleophile replaces a leaving group (common in haloalkanes).

Nucleoside: A compound consisting of a nitrogenous base and a sugar (without phosphate).

Nucleotide: The monomer of nucleic acids (Base + Sugar + Phosphate).

Nucleus: It is the small positively charged part of the atom. It is situated at the center. It contains neutron and protons collectively called as nucleons.

O

Octahedral Void: It is surrounded by six closely placed spheres, four are placed in same plane. One is above and the other is below the plane.

Octet Rule: Atoms tend to gain, lose, or share electrons to achieve a stable configuration of 8 valence electrons.

Optical Activity: The ability of a chiral substance to rotate plane-polarized light.

Optical Isomers (Enantiomers): Stereoisomers that are non-superimposable mirror images.

Orbital: A 3D region of space around the nucleus where the probability of finding an electron is high (>90 %).

Order of Reaction: The sum of the powers of concentration terms in the rate law.

Ore: A naturally occurring solid material from which a metal or valuable mineral can be extracted profitably.

Organic Chemistry: The branch of chemistry studying carbon-containing compounds.

Osmosis: The flow of solvent molecules through a semipermeable membrane from a dilute solution to a concentrated one.

Osmotic Pressure: The pressure required to stop osmosis (π = CRT).

Ostwald’s Dilution Law: For a weak electrolyte, the degree of ionization increases with dilution.

Oxidation: The loss of electrons or increase in oxidation state.

Oxidation Number (State): The theoretical charge an atom would have if all bonds were ionic.

Oxidizing Agent (Oxidant): A substance that gains electrons (is reduced) and helps another substance oxidize.

Oxoacid: An acid containing hydrogen, oxygen, and another element (e.g., H₂SO₄).

Ozonolysis: Reaction of alkenes with ozone (O₃) to break the double bond and form carbonyl compounds.

P

Paramagnetic Substances: The substances which are weakly attracted when placed in the external magnetic field and the property is called as paramagnetism.

Partial Pressure: The pressure exerted by an individual gas in a mixture.

Pauli Exclusion Principle: No two electrons in an atom can have the same set of all four quantum numbers. It means if an orbital has two electrons they must have opposite spin.

Peptide Bond: The amide linkage (-CONH-) between amino acids in a protein.

Peptization: The process of converting a precipitate into a colloid by shaking it with an electrolyte.

Period: A horizontal row in the periodic table.

Periodicity: The recurring trends in element properties with increasing atomic number.

pH: The negative logarithm of the hydrogen ion concentration (pH = -log[H⁺]).

Phase: A distinct and homogeneous form of matter (solid, liquid, or gas).

Phenol: An aromatic compound with an -OH group bonded directly to the benzene ring.

Phosphodiester Linkage: The bond holding the sugar-phosphate backbone of DNA/RNA together.

Photoelectric effect: When a beam of light of high frequency is allowed to strike a metal surface in vacuum, electrons are ejected from the metal surface. The phenomenon is known as photoelectric effect and the electrons are photoelectrons.

Photons: Light consists of a number of particles (packets of electromagnetic energy) of no mass called as photons.

Pi-Bond: A covalent bond formed by the sideways overlap of p-orbitals.

Planck’s Constant (h): 6.626 x 10^-34 J/s.

pOH: The negative logarithm of the hydroxide ion concentration (pOH = -log[OH^–]).

Polar Covalent Bond: A bond with unequal sharing of electrons.

Polarimeter: An instrument used to measure the angle of rotation of plane-polarized light.

Polymer: A large molecule made of repeating subunits (monomers).

Polysaccharide: Carbohydrates made of long chains of monosaccharides (e.g., Starch, Cellulose).

Positron: It is the positive counterpart of electron. It is very unstable and symbolised as ₁e⁰ or e^t.

Potential Energy: Stored energy based on position or structure.

Precipitate: An insoluble solid formed from a reaction in a solution.

Primary Structure (Protein): The specific sequence of amino acids in a polypeptide chain.

Principal Quantum Number: This number gives the average distance of the electron from the nucleus and corresponds to the principal energy level to which the electron belongs. Represented by ‘n’. The maximum number of electrons in ‘n’ principal quantum number is given by 2n².

Proton: The fundamental particle of the anode ray particles having positive charge and also which is an integral multiple of electronic charge and a specific charge to mass ratio, both depending on the nature of the gas in discharge tube are known as protons. Charge of proton is +1.6 x 10^-19 C and mass of proton is 1.67 x 10^-27 Kg.

Pseudo-First Order Reaction: A reaction that is bimolecular but follows first-order kinetics because one reactant is in excess.

Q

Qualitative Analysis: Analysis to determine what is present in a sample.

Quantitative Analysis: Analysis to determine how much is present.

Quantization: The concept that energy comes in discrete packets (quanta) rather than a continuous flow.

Quantum Mechanics: The branch of physics describing the behavior of matter at the atomic/subatomic level.

Quantum Numbers: Four numbers (n, l, m, s) specifying the state of an electron.

Quaternary Structure: The arrangement of multiple folded protein subunits in a multi-subunit complex.

Quenching: Rapid cooling of a substance to obtain certain properties.

R

Racemic Mixture: A 50:50 mixture of enantiomers that is optically inactive.

Radioactivity: The spontaneous emission of radiation by an unstable atomic nucleus.

Raoult’s Law: The vapor pressure of a solution component is the product of its mole fraction and the vapor pressure of the pure solvent.

Rate Constant (k): The proportionality constant in the rate law equation.

Rate Law: An expression relating the rate of a reaction to the concentration of reactants.

Rate-Determining Step: The slowest step in a reaction mechanism.

Reactant: A substance that undergoes change during a reaction.

Real Gas: It is defined as a gas which obeys the gas laws under low pressure or high temperature.

Redox Reaction: A reaction involving the transfer of electrons (Reduction + Oxidation).

Reducing Agent (Reductant): A substance that donates electrons (is oxidized).

Reducing Sugar: A sugar capable of acting as a reducing agent (has free aldehyde/ketone group).

Reduction: The gain of electrons or decrease in oxidation number.

Reduction Potential: The tendency of a chemical species to acquire electrons.

Resonance: The delocalization of electrons in molecules that cannot be represented by a single Lewis structure.

Retention of Configuration: When the spatial arrangement around a chiral center is preserved during a reaction.

Reversible Reaction: A reaction that can proceed in both forward and backward directions.

Roasting: Heating an ore (usually sulphide) in excess air to convert it to oxide.

Root Mean Square Velocity: It is the square root of the mean of squares of the velocity of a large number of molecules of the same gas.

Rosenmund Reduction: Conversion of acid chlorides to aldehydes using H₂/Pd-BaSO₄.

Rusting: The corrosion of iron to form hydrated iron(III) oxide.

Rutherford’s model of atom: An atom consists of a positively charged heavy central core, called as nucleus. The size of the nucleus is very small as compared to the total size of the atom. The electrons revolve around the nucleus and their total negative charge is equal to the positive charge of the nucleus, so that the atom as a whole is electrically neutral so they form the extra-nuclear region of the atom.

Rydberg Constant: A physical constant relating to the atomic spectra of hydrogen.

S

Salt Bridge: A tube containing an electrolyte that connects the two half-cells of a galvanic cell to maintain electrical neutrality.

Saponification: The hydrolysis of an ester (fat/oil) by a strong base to produce soap and glycerol.

Saturated Solution: A solution containing the maximum amount of solute that can dissolve at a given temperature.

Second: It is defined as the duration of 9,192,631,770 cycles of the radiation associated with specified transition of Cesium-133.

Second Law of Thermodynamics: The total entropy of an isolated system can never decrease over time.

Secondary Structure (Protein): Local folding of the polypeptide chain into alpha-helices or beta-sheets.

Semiconductor: A material with conductivity between a conductor and an insulator (e.g., Silicon).

Semipermeable Membrane: A membrane that allows solvent molecules to pass but blocks solute particles.

Shielding Effect: The repulsion of outer electrons by inner electrons, reducing the nuclear pull.

Sigma Bond: A strong covalent bond formed by head-on overlap of orbitals.

Significant Figures: The digits in a number that carry meaning contributing to its measurement resolution.

S_N1 Reaction: Unimolecular nucleophilic substitution (two steps, carbocation intermediate, racemization).

S_N2 Reaction: Bimolecular nucleophilic substitution (one step, transition state, inversion of configuration).

Solubility: The maximum amount of solute that can dissolve in a solvent at a specific temperature.

Solubility Product (K_sp): The equilibrium constant for the dissolution of a sparingly soluble salt.

Solute: The substance dissolved in a solution.

Solvent: The substance in which the solute dissolves.

Specific Heat: It is defined as the quantity of heat required to raise the temperature of 1 g of that substance through 1 °C.

Spectrochemical Series: A list of ligands ordered by the strength of the field they produce.

Spin Quantum Number: This number arises due to the spinning of the electron about its own axis. Represented by ‘s’. The spin may be clockwise represented by +¹/₂ and anticlockwise by –¹/₂.

Spontaneous Process: A process that occurs naturally without external intervention (ΔG < 0).

Standard Enthalpy of Formation: The heat change when 1 mole of a substance is formed from its elements in their standard states.

Standard Hydrogen Electrode (SHE): The reference electrode assigned a potential of 0.0 V.

Stark effect: It is the splitting of spectral lines into a group of finer lines under the influence of electric field.

State Function: A property whose value depends only on the current state of the system, not the path taken (e.g., H, S, G).

Stereoisomerism: Isomerism where atoms have the same connectivity but different spatial arrangement.

Stoichiometry: The calculation of reactants and products in chemical reactions.

Strong Electrolyte: A substance that completely dissociates into ions in solution.

Sublimation: The transition of a substance directly from solid to gas phase.

Substrate: The molecule upon which an enzyme acts.

Surface Tension: The energy required to increase the surface area of a liquid.

Surroundings: Everything outside the thermodynamic system.

Suspension: A heterogeneous mixture containing solid particles large enough for sedimentation.

T

Temperature: A measure of the average kinetic energy of particles.

Tertiary Structure (Protein): The overall 3D shape of a protein molecule.

Tetrahedral Void: It is developed when triangular voids have contact with one sphere either in the upper layer or in the lower layer.

Theoretical Yield: The maximum amount of product that can be generated from a reaction (calculated).

Thermodynamics: The study of energy, heat, and work.

Threshold frequency: The certain minimum energy of which can just cause the ejection of electrons.

Third Law of Thermodynamics: The entropy of a perfect crystal at absolute zero is zero.

Thomson’s Model of atom: An atom is a sphere of positive charges with negative charges embedded in it, such that the atom as a whole remains neutral. This model is also known as water-melon model of atom. But this model could not explain the Rutherford’s gold-foil scattering experiment.

Titration: A technique to determine the concentration of a solution by reacting it with a standard solution.

Tollen’s Reagent: Ammoniacal silver nitrate; used to test for aldehydes (Silver Mirror Test).

Trans-Isomer: A geometric isomer where similar groups are on opposite sides of the double bond.

Transition Elements: D-block elements that have partially filled d-orbitals.

Transition State: A high-energy state corresponding to the highest potential energy along the reaction coordinate.

Trigonal Void: It is formed when three spheres lie at the vertices of an equilateral triangle.

Triple Bond: A bond involving the sharing of three pairs of electrons (one sigma, two pi).

Tyndall Effect: The scattering of a light beam as it passes through a colloid.

U

Unimolecular Reaction: An elementary reaction involving a single molecule.

Unit Cell: The smallest repeating unit of a crystal lattice.

Unsaturated Hydrocarbon: A hydrocarbon containing double or triple bonds.

Unsaturated Solution: A solution that can dissolve more solute at the current temperature.

V

Valence Bond Theory (VBT): Explains bonding via the overlap of atomic orbitals.

Valence Electrons: Electrons in the outermost shell of an atom.

Valency: The combining capacity of an element.

Van der Waals Forces: Weak intermolecular attractive forces.

Van der Waal’s Radius: Half of the internuclear distance between two nearest atoms belonging to two adjacent molecules in solid state.

Van’t Hoff Factor (i): Ratio of observed colligative property to theoretical value; measures dissociation/association.

Vapor Pressure: The pressure exerted by a vapor in equilibrium with its liquid or solid phase.

Velocity: The distance travelled by wave in one second.

Viscosity: A measure of a fluid’s resistance to flow.

Volatile: A substance that evaporates readily at normal temperatures.

Voltage: Electrical potential difference.

Volume: The amount of 3D space a substance occupies.

W

Water of Crystallization: Water molecules that form part of the crystal structure of a salt (e.g., CuSO₄.5H₂O).

Wavelength: The distance between two neighbouring troughs or crests. It determines the colour of visible light.

Wavenumber: It is defined as the number of wavelength per cm, and is equal to the inverse of wavelength expressed in centimetres.

Weak Electrolyte: An electrolyte that dissociates only partially in solution.

Williamson Synthesis: Synthesis of ethers by reacting an alkyl halide with a sodium alkoxide.

Work: Energy transfer due to a force moving an object (In Chem: W = -PΔV).

Wurtz Reaction: Reaction of alkyl halides with sodium in dry ether to form higher alkanes.

X

X-Ray Diffraction: A technique used to determine the atomic and molecular structure of a crystal.

Y

Yield: The amount of product obtained in a reaction.

Z

Zeeman effect: It is the splitting of spectral lines into a group of finer lines under the influence of magnetic field.

Zeolites: Microporous aluminosilicates used as catalysts (shape-selective catalysis).

Zero Order Reaction: A reaction whose rate is independent of reactant concentration.

Zinc Dust: Powdered zinc used as a reducing agent (e.g., reducing Phenol to Benzene).

Zwitterion: A dipolar ion with spatially separated positive and negative charges (common in amino acids).

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