A Level Chemistry Glossary

A comprehensive set of vocabulary flashcards covering key concepts in A Level Chemistry.

Atomic Number

The number of protons in the nucleus of an atom.

Mass Number

The number of protons and neutrons in the nucleus of an atom.

Isotopes

Atoms of the same element with different numbers of neutrons and different masses.

Atomic Orbital

A region around the nucleus that can hold up to two electrons, with opposite spins.

p-Block Element

Element which has the highest energy electron in a p sub-shell.

First Ionisation Energy

The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

Successive Ionisation Energy

A measure of the energy required to remove each electron in turn.

Molar Mass

The mass, in grams, per mole of a substance.

Relative Isotopic Mass

The mass of an atom of an isotope compared with 1/12 of the mass of an atom of Carbon-12.

Relative Atomic Mass

The weighted mean mass of an atom of an element compared with 1/12 of the mass of an atom of Carbon-12.

Mole

The amount of substance containing as many particles as there are carbon atoms in exactly 12g of Carbon-12.

Avogadro’s Constant, N_A

The number of particles per mole of a substance (6.02 x 10²³ mol⁻¹).

Molar Gas Volume

The gas volume per mole of a substance, which is 24.0 dm³mol⁻¹ at room temperature and pressure.

Empirical Formula

The simplest whole number ratio of atoms of each element present in a compound.

Molecular Formula

The actual number and type of atoms of each element in a molecule.

Water of Crystallisation

The water present in a compound giving the compound a crystalline appearance.

Anhydrous

When all the waters of crystallisation have been removed from a compound.

Hydrated

When water of crystallisation is present in a crystal compound.

Ionic Bond

The electrostatic attraction between positive and negative ions.

Covalent Bond

The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.

Dative Covalent Bond

A bond where only one of the atoms supplies both of the shared electrons.

Electronegativity

The ability of an atom to attract the bonding electrons towards itself in a covalent bond.

Electron Pair Repulsion Theory

The shape of simple covalent molecules and ions are determined by the number & type of electron pairs around the central atom.

Metallic Bonding

The strong electrostatic attraction between metal cations and delocalised electrons.

Periodicity

Repeating trends in physical and chemical properties across different periods.

Enthalpy Change

The amount of heat released or absorbed by a chemical reaction at constant pressure.

Exothermic Reaction

A reaction where heat energy is released to the surroundings.

Endothermic Reaction

A reaction where heat energy is absorbed from the surroundings.

Hess’ Law

The enthalpy change of a reaction depends only on the initial and final states.

Average Bond Enthalpy

The average enthalpy change for the breaking of one mole of bonds in gaseous molecules.

Standard Solution

A solution of known concentration.

Standard Conditions

Specific conditions: temperature at 25°C, pressure of gases at 1 atm, concentration of aqueous solutions at 1.00 mol dm⁻³.

Standard Enthalpy Change of Reaction

The enthalpy change that accompanies a reaction under standard conditions.

Standard Enthalpy Change of Neutralisation

The enthalpy change when an acid is neutralised by a base to form one mole of water.

Standard Enthalpy Change of Combustion

The enthalpy change when one mole of a substance reacts by complete combustion.

Lattice Enthalpy

The enthalpy change when one mole of an ionic compound is formed from its gaseous ions.

Standard Enthalpy Change of Hydration

The enthalpy change when one mole of gaseous ions is dissolved in water.

Entropy

A measure of the dispersal of energy in a system.

Gibbs Free Energy Change (ΔG)

The balance between enthalpy change, entropy change, and temperature for a process.

Rate of Reaction

The change in concentration of a reactant or product per unit time.

Rate Equation

Gives the relationship between the rate of reaction and the concentration of reactants.

Rate Constant (k)

The constant in the rate equation that indicates the speed of a reaction.

Order

How the concentration of a reactant affects the rate of the reaction.

Activation Energy

The minimum energy required for a reaction to take place.

Catalyst

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

Homogeneous Catalyst

A catalyst in the same physical state as the reactants.

Heterogeneous Catalyst

A catalyst in a different physical state from the reactants.

Rate-Determining Step

The slowest step of a reaction mechanism.

Half-Life

The time taken for the concentration of a reactant to fall to half its original concentration.

Dynamic Equilibrium

When the rate of the forward reaction equals the rate of the reverse reaction.

Le Chatelier’s Principle

When a system in equilibrium is subjected to a change, the position of equilibrium will shift to minimize the effects of the change.

Equilibrium Constant (Kc)

A constant that expresses the ratio of the concentrations of products to reactants at equilibrium.

Acid (Bronsted-Lowry)

A proton donor.

Base (Bronsted-Lowry)

A proton acceptor.

Alkali

A base that dissolves in water and releases OH⁻ ions.

Salt

A compound produced when a H⁺ ion from an acid is replaced by a metal or another positive ion.

Conjugate Acid-Base Pair

A pair that transforms into each other by the gain or loss of a proton.

Strong Acid

An acid that completely dissociates into ions in solution.

Weak Acid

An acid that partially dissociates into ions in solution.

Monobasic Acid

An acid that donates one mole of protons per mole of acid.

pH

A measure of acidity or alkalinity of a solution.

Acid Dissociation Constant (Ka)

The equilibrium constant for the dissociation of a weak acid.

Ionic Product of Water (Kw)

The equilibrium constant for the dissociation of water.

Buffer Solution

A solution that resists changes in pH when small amounts of acid or base are added.

Oxidation Number

A measure of the number of electrons an atom uses to bond.

Oxidation

The loss of electrons or an increase in oxidation number.

Reduction

The gain of electrons or a decrease in oxidation number.

Oxidising Agent

A reagent that oxidises another species and is itself reduced.

Reducing Agent

A reagent that reduces another species and is itself oxidised.

Redox Reaction

A reaction where both oxidation and reduction occur.

Displacement Reaction

A reaction where a more reactive element displaces a less reactive one.

Disproportionation

A reaction in which the same element is both oxidised and reduced.

Standard Electrode Potential (Eθ)

The e.m.f. of a half cell compared with a standard hydrogen half-cell.

Standard Cell Potential (Eθcell)

The potential difference in a cell, calculated as Eθreduction - Eθoxidation.

Fuel Cell

A cell that uses the energy from fuel and oxygen to generate voltage.

d-Block Element

Element which has the highest energy electron in a d sub-shell.

Transition Element

A d block element with incomplete d sub-shell.

Ligand

A molecule or ion that can donate a pair of electrons to a transition metal ion.

Complex Ion

A transition metal ion bonded to one or more ligands by coordinate bonds.

Coordination Number

The total number of coordinate bonds formed between a metal ion and its ligands.

Bidentate Ligand

A ligand that can form two coordinate bonds with a metal ion.

Ligand Substitution

A reaction where one ligand in a complex ion is replaced by another ligand.

Hydrocarbon

A compound containing only hydrogen and carbon.

Homologous Series

A series of organic compounds with the same functional group differing by CH₂.

Structural Isomers

Compounds with the same molecular formula but different structural formulae.

Stereoisomers

Compounds with the same structural formula but different spatial arrangements.

E/Z Isomerism

A type of stereoisomerism involving restricted rotation about a double bond.

Cis/Trans Isomerism

A type of E/Z isomerism with identical substituents on each carbon of a double bond.

Optical Isomerism

Stereoisomers that are non-superimposable mirror images.

Chiral Centre

A carbon atom with four different groups attached.

Radical

A species with an unpaired electron.

Curly Arrow

A notation used to show the movement of an electron pair.

Electrophile

An electron pair acceptor.

Nucleophile

An electron pair donor.

Substitution Reaction

A reaction where an atom or group is replaced by another.

Addition Reaction

A reaction where a group is added across a double bond.

Elimination Reaction

A reaction where an atom or group is removed from a molecule.

Oxidation Reaction

A reaction where oxygen is added and/or hydrogen is removed.

Condensation Reaction

A reaction where H₂O is lost when two molecules join.

Dehydration Reaction

A reaction where H₂O is removed from a saturated molecule.