Formation of Ions — Ionic Bonding | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable

Medium

Stable9%

Ion formation, lattice structures and properties of ionic compounds tested consistently.

What an ion is

An ion is a charged atom (or group of atoms) formed when an atom loses or gains electrons
The number of electrons transferred equals the size of the charge:
- lose 1 electron → 1+ charge
- lose 2 electrons → 2+ charge
- gain 1 electron → 1− charge
- gain 2 electrons → 2− charge

Cations and anions

A cation is a positive ion, formed when an atom loses electrons (it now has fewer electrons than protons)
An anion is a negative ion, formed when an atom gains electrons (it now has more electrons than protons)
The driving force in every case is to reach a full outer shell — the stable electron arrangement of a noble gas
- Metals (1–3 outer electrons) lose those few electrons to form cations
- Non-metals (5–7 outer electrons) gain electrons to form anions
Group 4 elements rarely form simple ions (losing or gaining 4 electrons costs too much energy)
Group 0 noble gases already have full outer shells and so do not form ions

Predicting ion charges from group number

Group	Outer electrons	Ion charge
1	1	1+
2	2	2+
3	3	3+

Exam tip

Describing electron changes when ions form

What comes up: "Describe the changes in electronic configuration of [metal] and [non-metal] atoms to form the ions in [compound]."

Write (two marks): (1) State that the metal atom loses the correct number of electrons (e.g. each sodium atom loses one electron, changing from 2,8,1 to 2,8). (2) State that the non-metal atom gains the correct number of electrons (e.g. the oxygen atom gains two electrons, changing from 2,6 to 2,8).

Watch out: Any mention of sharing electrons scores zero for this type of question — describe electron transfer (loss and gain), never sharing. Name each atom separately and state the specific number of electrons it loses or gains.