Know how atoms may form positive ions by losing electrons or form negative ions by gaining electrons

5.1.1 The Atom

Objective

Know how atoms may form positive ions by losing electrons or form negative ions by gaining electrons.

Atoms – basic structure

An atom consists of a nucleus containing protons (\$p^{+}\$) and neutrons (\$n^{0}\$) surrounded by electrons (\$e^{-}\$) in shells.

Ions – charged atoms

When the number of electrons is not equal to the number of protons, the atom carries a net electric charge and is called an ion.

• Fewer electrons than protons → positive ion (cation).
• More electrons than protons → negative ion (anion).

Formation of positive ions (cations)

Atoms lose one or more electrons. The loss requires energy (ionisation energy) and may be facilitated by collisions, high temperature, or chemical reactions.

General equation:

\$\text{X} \rightarrow \text{X}^{n+} + n\,e^{-}\$

where \$n\$ is the number of electrons removed.

Formation of negative ions (anions)

Atoms gain one or more electrons. The gain releases energy (electron affinity) and often occurs when an atom has a high affinity for electrons.

General equation:

\$\text{X} + n\,e^{-} \rightarrow \text{X}^{n-}\$

Examples

Key points to remember

1. Atoms are electrically neutral when the number of protons equals the number of electrons.
2. Loss of electrons → positive ion; gain of electrons → negative ion.
3. The magnitude of the charge equals the number of electrons lost or gained.
4. Ionisation energy is required to remove electrons; electron affinity is released when electrons are added.
5. Metals tend to form cations; non‑metals tend to form anions.

Practice questions

1. Write the ionic symbol for an aluminium atom that has lost three electrons.
2. What charge will a sulphur atom have after gaining two electrons?
3. Explain why sodium readily forms a \$+\$1 ion while chlorine readily forms a \$-\$1 ion.