Comprehensive AS and A2 Chemistry Definitions Guide

Isotope

Atoms of the same element with the same number of protons and electrons but different number of neutrons and different masses.

Relative atomic mass ($A_r$)

The weighted mean mass of an atom of an element compared with $1/12^{th}$ the mass of one atom of carbon-12.

Mole

Unit for amount of substance. It contains Avogadro’s constant of $6.02 \times 10^{23}$ particles.

Avogadro’s constant ($N_A$)

Number of atoms per mole = $6.02 \times 10^{23} \, mol^{-1}$.

Orbital

A region around the nucleus that can hold up to $2$ electrons with opposite spins.

First ionisation energy

Energy needed to remove $1 \, mol$ of electrons from $1 \, mol$ of gaseous atoms to form $1 \, mol$ of gaseous $1+$ ions.

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.

Hydrated

A crystalline compound containing water of crystallisation.

Anhydrous

Containing no water of crystallisation.

Acid

A substance which releases $H^+$ ions in aqueous solution.

Base

A substance which accepts $H^+$ ions from an acid.

Alkali

A substance that releases $OH^-$ ions in aqueous solution.

Salt

$H^+$ ions have been replaced by the metal ions or ammonium ion.

Oxidation

Loss of electrons. Oxidation number increases.

Reduction

Gain of electrons. Oxidation number decreases.

Ionic bonding

Strong electrostatic attraction between positive and negative ions.

Covalent bonding

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

Sigma bond ()

Overlap of orbitals directly between the bonding atoms.

Electronegativity

Ability of an atom to attract the bonding electrons in a covalent bond.

Dynamic equilibrium

Exists in a closed system where the rate of the forward reaction equals the rate of the reverse reaction.

Bronsted-Lowry acid

Proton donor.

Bronsted-Lowry base

Proton acceptor.

Homologous series

A series of organic compounds having the same functional group but differing by $CH_2$.

Condensation polymerisation

Joining monomers to form a polymer with the loss of a small molecule.