1.3 Electrons, energy levels and atomic orbitals

Define the term electronic configuration.

The arrangment of electrons in an atom.

Describe how electrons are arranged in an atom.

Electrons are arranged around the nucleus in principal quantum shells.

State what is meant by the principal quantum number (n).

It labels the energy levels; the higher the number, the higher the energy and the farther the shell is from the nucleus.

State the maximum number of electrons each principal quantum shell can hold.

• n = 1 → 2 electrons.
• n = 2 → 8 electrons.
• n = 3 → 18 electrons.
• n = 4 → 32 electrons.

Describe the relationship between principal shells and sub-shells.

Each principal shell is divided into sub-shells with increasing energy: s < p < d < f.

State the number of orbitals and electrons each sub-shell can hold.

• s: 1 orbital, 2 electrons.
• p: 3 orbitals, 6 electrons.
• d: 5 orbitals, 10 electrons.
• f: 7 orbitals, 14 electrons.

Identify the sub-shells present in the first three principal shells.

• Shell 1: 1s.
• Shell 2: 2s, 2p.
• Shell 3: 3s, 3p, 3d.

Explain why electrons fill lower energy orbitals first.

To make the atom more stable by occupying orbitals in order of increasing energy.

State the order of orbital energy levels from lowest to highest.

s < p < d < f.

Explain what causes electron repulsion in orbitals.

Electrons are negatively charged and repel each other.

Describe how electrons reduce repulsion within orbitals.

They pair with opposite spins (clockwise and anticlockwise).

Explain why electrons occupy separate orbitals before pairing.

To minimise repulsion within the same sub-shell.

State how anions are formed.

By gaining electrons in the outer shell.

State how cations are formed.

By losing electrons from the outer shell.

Describe how electrons are gained or lost in transition metals.

• Fill 4s before 3d when neutral.
• Lose electrons from 4s first when forming ions.

Describe the steps used to write a shorthand electronic configuration.

1. Write the full configuration.

2. Find the previous noble gas.

3. Replace the first part with the noble gas symbol.

4. Write the remaining part after the noble gas.

Give the shorthand configuration of chlorine (Cl).

[Ne] 3s² 3p⁵.

State the number of boxes used in electron box notation for s, p, and d sub-shells.

• s → 1 box.
• p → 3 boxes.
• d → 5 boxes.

Define the term free radical.

A species with one or more unpaired electron.