Describe the formation of positive ions, known as cations, and negative ions, known as anions

Atoms, Elements and Compounds – Ions and Ionic Bonds

Learning objective

Describe how positive ions (cations) and negative ions (anions) are formed, and explain how these ions combine to give the characteristic properties of ionic compounds.

What is an ion?

An ion is an atom or a group of atoms that has gained or lost one or more electrons, giving it a net electrical charge.

Why do ions form?

• Atoms tend to achieve the electron configuration of the nearest noble gas.
• Metals lose electrons because their ionisation energies are relatively low.
• Non‑metals gain electrons because their electron affinities are relatively high.
• The resulting opposite charges attract each other electrostatically, producing an ionic bond.

Typical ionic charges (core syllabus)

• Group I elements → +1
• Group II elements → +2
• Group III elements (main‑group) → +3
• Group VII elements → –1
• Group VI elements → –2
• Group V elements → –3

These charges correspond to the number of electrons lost (cations) or gained (anions) to reach a noble‑gas configuration.

Formation of cations (positive ions)

• Metals have low ionisation energies, so they lose electrons.
• Loss of electrons gives a net positive charge; the magnitude equals the number of electrons lost.
• Typical charges are +1, +2 or +3.

Balanced half‑reactions:

$$\text{Na} \;\rightarrow\; \text{Na}^{+} + e^{-}$$

$$\text{Mg} \;\rightarrow\; \text{Mg}^{2+} + 2e^{-}$$

$$\text{Al} \;\rightarrow\; \text{Al}^{3+} + 3e^{-}$$

Formation of anions (negative ions)

• Non‑metals have high electron affinities, so they gain electrons.
• Gain of electrons gives a net negative charge; the magnitude equals the number of electrons gained.
• Typical charges are –1, –2 or –3.

Balanced half‑reactions:

$$\text{Cl} + e^{-} \;\rightarrow\; \text{Cl}^{-}$$

$$\text{O} + 2e^{-} \;\rightarrow\; \text{O}^{2-}$$

$$\text{N} + 3e^{-} \;\rightarrow\; \text{N}^{3-}$$

Dot‑and‑cross diagrams (required by the syllabus)

These diagrams show the transfer of electrons between atoms.

Example – formation of NaCl

Na •   →   Na⁺   +   •e⁻
Cl •   +   •e⁻   →   Cl⁻ •

Example – formation of MgCl₂

Mg • •   →   Mg²⁺   +   2•e⁻
Cl •   +   •e⁻   →   Cl⁻ •   (×2)

Example – formation of Al₂S₃

Al • • •   →   Al³⁺   +   3•e⁻   (×2)
S • •   +   2•e⁻   →   S²⁻ • •   (×3)

Class activity: Students draw their own dot‑and‑cross diagrams for the compounds above and for KBr.

Charge balance – rule for neutral ionic compounds

Examples of charge‑balanced formulas

• NaCl: Na⁺ (1⁺) + Cl⁻ (1⁻) → neutral.
• MgCl₂: Mg²⁺ (2⁺) + 2 Cl⁻ (2 × 1⁻) → neutral.
• Al₂S₃: 2 Al³⁺ (2 × 3⁺) + 3 S²⁻ (3 × 2⁻) → neutral.
• KBr: K⁺ (1⁺) + Br⁻ (1⁻) → neutral.

Giant lattice structure

Ionic solids consist of a three‑dimensional array of alternating cations and anions held together by strong electrostatic attractions. This “giant lattice” explains the high melting points, brittleness and the need for the lattice to be broken before ions can move.

Properties of ionic compounds (core requirement)

• High melting and boiling points – due to the strong electrostatic forces in the giant lattice.
• Brittle solids – when a layer of ions shifts, like‑charged ions repel and the crystal fractures.
• Electrical conductivity only when the lattice is broken (molten state or aqueous solution), because the ions are then free to move.
• Solubility in water – polar water molecules can separate the ions, allowing them to conduct electricity in solution.

Typical ions of common elements

Key points to remember

• Cations are formed by loss of electrons; anions are formed by gain of electrons.
• The magnitude of the charge equals the number of electrons lost (positive) or gained (negative).
• Metals → cations; non‑metals → anions.
• When writing formulas, the total positive charge must equal the total negative charge.
• Ionic compounds consist of a giant lattice of oppositely charged ions; this lattice accounts for their high melting points, brittleness, and conductivity only when the lattice is broken (melt or solution).
• Typical ionic charges can be predicted directly from the element’s group number (core syllabus).