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

Relative Isotopic Mass

The mass of an isotope relative to 1/12th the mass of an atom of carbon-12.

Relative Atomic Mass

The weighted mean mass of an element is the weighted mean mass of an atom of an element, relative to one-twelfth (1/12) the mass of carbon-12.

Empirical Formula

The empirical formula is the simplest whole number ratio of atoms of each atom present in a compound.

Anhydrous Salt

A salt in which there are no waters of crystallisation.

Hydrated Salt

A salt containing water of crystallisation.


An acid is a proton donor.


A base is a proton acceptor.


A compound where the hydrogen ions from an acid have been replaced by metal ions or ammonium ions.

First Ionisation Energy

The first ionisation energy is the amount of energy needed to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ atoms.

Atomic Orbital

A region within an atom that is able to hold up to two electrons, with opposite spins.


The amount of substance which contains as many particles as there are carbon atoms in 12g of carbon-12 atoms.

Octet rule

The tendency to acquire a noble gas configuration.

Metallic Bonding

A bond formed by the attraction between positive metal ions and a sea of delocalised electrons.

Ionic Bonding

Electrostatic attraction between oppositely charged ions.

Covalent Bonding

A shared pair of electrons.

Dative Covalent Bond

A bond formed by a shared pair of electrons where both electrons have been donated by one atom.


The ability of an atom to attract electrons in a covalent bond.


The repeating pattern of properties shown across different periods.

Disproportionation Reaction

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

Giant Ionic Lattice

A three dimensional structure of oppositely charged ions held together by strong ionic bonds.

© Andrew Deniszczyc, 2023