Atomic structure and the periodic table: every key term you need (+ practice quiz)

Isotope

Atoms of the same element with equal proton number but different neutron numbers. Chemically identical, since chemistry depends on electrons.

Relative atomic mass

The weighted mean mass of an element's atoms relative to one twelfth the mass of a carbon-12 atom.

Time of flight mass spectrometer

Ionises a sample, accelerates all ions to the same kinetic energy, then times their flight. Lighter ions arrive first.

Electron sub-shell

Regions labelled s, p, d and f holding 2, 6, 10 and 14 electrons. Filling order explains the shape of the periodic table.

First ionisation energy

Energy to remove one mole of electrons from one mole of gaseous atoms, forming 1+ ions. Falls down a group as shielding and radius increase.

Shielding

Inner-shell electrons repel outer electrons, reducing the effective nuclear charge felt by them. A key driver of periodic trends.

Ionisation energy anomalies

The small drops between groups 2 and 3, and 5 and 6, reveal sub-shell structure — a new p sub-shell starting, then electron pairing repulsion.

Successive ionisation energies

Each removal is harder than the last. A large jump reveals a new shell being broken into, identifying the group.

Atomic radius trend

Decreases across a period as nuclear charge rises with no extra shielding; increases down a group as shells are added.

Periodicity

The repeating pattern of properties across successive periods, arising from the repeating pattern of electron configuration.

Period 3 oxides

Change from ionic and basic on the left, through amphoteric aluminium oxide, to covalent and acidic on the right.

Group 2 reactivity

Increases down the group: atomic radius rises and shielding increases, so the outer electrons are lost more easily.

Group 7 reactivity

Decreases down the group. Larger atoms with more shielding attract an incoming electron less strongly, so oxidising power falls.

Disproportionation

One species is simultaneously oxidised and reduced. Chlorine in water is the standard example.

Flame test

Electrons absorb energy and jump to higher levels, then emit a characteristic wavelength as they fall back, giving each ion its colour.

Transition metal definition

An element forming at least one stable ion with a partially filled d sub-shell. This gives variable oxidation states and coloured ions.

More A-Level Chemistry guides

• Bonding, structure and intermolecular forces
• Amount of substance and quantitative chemistry
• Energetics and thermodynamics
• Kinetics, equilibria and acids
• Redox, electrochemistry and transition metals