Part 3: Electron configurations

1. Shells, sub-shells and orbitals

Electrons are arranged in shells (principal energy levels, n = 1, 2, 3…). Each shell is split into sub-shells (s, p, d), and each sub-shell contains orbitals.

An orbital is a region of space where there is a high probability of finding an electron. Each orbital can hold a maximum of 2 electrons, with opposite spins.

Shell 1 has only 1s. Shell 2 has 2s and 2p. Shell 3 has 3s, 3p and 3d. Shell 4 starts with 4s and 4p (and 4d and 4f beyond Kr).

2. Filling order and rules

For atoms up to krypton, sub-shells fill in this order:

1s 2s 2p 3s 3p 4s 3d 4p

• Lowest energy first: electrons go into the lowest-energy available sub-shell.
• Single first, then pair: within orbitals of equal energy (e.g. the three 2p orbitals), electrons go in one per orbital with parallel spins before any pairing. This is because paired electrons in the same orbital repel each other.
• Two per orbital, opposite spins: no orbital holds more than two electrons.

The rule "4s before 3d" describes the filling order of atoms. It does not mean 4s is always lower in energy: once the 3d orbitals are occupied, 4s electrons are the first to be removed when transition metal ions form.

3. Electrons-in-boxes

4. Writing configurations

In 3p⁴, the 3 is the shell, p is the sub-shell, and the superscript 4 is the number of electrons in that sub-shell. Check: the superscripts add up to the total number of electrons.

Example: sulfur (Z = 16) is 1s² 2s² 2p⁶ 3s² 3p⁴, and 2 + 2 + 6 + 2 + 4 = 16 ✓.

A shortened form uses the previous noble gas: [He] = 1s², [Ne] = 1s² 2s² 2p⁶, [Ar] = 1s² 2s² 2p⁶ 3s² 3p⁶. So sulfur is [Ne] 3s² 3p⁴. If a question asks for the full configuration, write out every sub-shell.

5. Configurations from H to Kr

6. Period 4 and the exceptions Cr and Cu

After argon, 4s fills first: K is [Ar] 4s¹ and Ca is [Ar] 4s². Next, 3d fills from Sc ([Ar] 3d¹ 4s²) to Zn ([Ar] 3d¹⁰ 4s²). Then 4p fills, ending at Kr ([Ar] 3d¹⁰ 4s² 4p⁶).

7. Configurations of ions

Start from the neutral atom, then add or remove electrons. The number of protons (Z) never changes.

• Main group: remove electrons from (or add them to) the outer sub-shell.
• Transition metals: remove the 4s electrons first, then 3d.

Worked: Fe³⁺. Fe is [Ar] 3d⁶ 4s² (26 electrons). Removing both 4s electrons and then one 3d electron leaves 23 electrons: [Ar] 3d⁵. Written in full, that is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵.

8. Blocks of the periodic table

An element's block depends on the sub-shell that its highest-energy electrons are filling:

• s block: Groups 1 and 2, plus helium (1s²)
• p block: Groups 13–18, except helium
• d block: the central transition-metal region (Sc to Zn in Period 4)

Don't decide the block from whatever sub-shell happens to be written last. For a neutral atom, the period is its outermost occupied shell. For a main-group element, its outer-shell electrons are the s and p electrons in that shell.

9. Finesse practice

Original Finesse practice questions, not AQA past-paper questions.

10. Sources