2.40620 · Section 2: Atoms, elements and compounds

Ions and ionic bonds

Key learnings

Ion charge follows the group number
Metals in Groups I, II and III lose electrons to form positive ions (cations) whose charge equals the group number: $\text{Na}^+$ , $\text{Mg}^{2+}$ , $\text{Al}^{3+}$ . Non-metals in Groups V, VI and VII gain electrons to form negative ions (anions), with charge equal to 8 minus the group number. Transition metals form variable-charge ions shown by a Roman numeral in the name, such as iron(II) and iron(III). Noble gases have a full outer shell and do not normally form ions.
Ionic formulae balance charges to zero
Build an ionic formula so the positive and negative charges cancel: cross-multiply each ion charge magnitude into the other subscript, then simplify to the smallest ratio — giving NaCl, $\text{MgCl}_2$ and $\text{Al}_2\text{O}_3$ . Learn the common polyatomic ions: sulfate and carbonate (2−), nitrate and hydroxide (1−), ammonium (1+). Bracket a polyatomic ion when more than one is needed, e.g. $\text{Ca(OH)}_2$ .
Giant ionic lattice explains the properties
Ionic compounds form a giant lattice: a regular 3-D array of oppositely charged ions held by strong electrostatic forces acting in all directions, with no molecules. The many strong attractions give high melting and boiling points. The solid does not conduct because the ions are fixed, but when molten or dissolved they are free to move and carry charge. The lattice is hard yet brittle — a blow aligns like charges, which repel and split the crystal.

Common mistakes

Drawn from real examiner reports.

Positive ions have fewer electrons, not more

A positive charge means electrons were lost, so a cation has fewer than the neutral atom — $\text{Na}^+$ has 10 electrons (2.8), not 11. A negative charge means electrons were gained, so an anion has more — $\text{Cl}^-$ has 18 (2.8.8), not 17. Check with: electrons = atomic number minus the charge (cation) or plus the charge magnitude (anion).

W22 Paper 13 Q4; W22 Paper 23 Q4; S23 Paper 41 Q3(a)(i) — candidates added an electron to Na+, showing it with 11 electrons.

Ion charge is not the mass number

The charge on an ion is separate from its mass number. In $^{23}_{11}\text{Na}^+$ the charge is 1+, not 23+, and a magnesium ion is $\text{Mg}^{2+}$ , not $\text{Mg}^{24}$ . The mass number counts protons plus neutrons; the charge counts electrons lost or gained, set by the group. Writing one where the other belongs loses the mark.

S22 Paper 31 Q1(b); S22 Paper 41 Q2(a) — candidates gave mass-number values as the ion charge.

Ionic bond is attraction, not sharing

An ionic bond is the electrostatic attraction between oppositely charged ions; both electrostatic and oppositely charged are needed. Do not write sharing a pair of electrons — that is a covalent bond. Do not write transfer of electrons either — transfer is how the ions form, not the bond itself. The bond is the attraction holding the finished ions together.

Conducts only when ions can move

Ionic compounds conduct when molten or dissolved, not when solid. The reason must mention that the ions are free to move: in the solid they are locked in the lattice and cannot carry charge, but melting or dissolving frees them to move to the electrodes. Saying it conducts when liquid, without the free-to-move idea, does not earn the explanation mark.

Bracket polyatomic ions in formulae

When a formula needs more than one polyatomic ion, bracket it: magnesium hydroxide is $\text{Mg(OH)}_2$ , not $\text{MgOH}_2$ . Also simplify the ratio after cross-multiplying — $\text{Ca}^{2+}$ with $\text{O}^{2-}$ gives CaO, not $\text{Ca}_2\text{O}_2$ . Forgetting a bracket or leaving the ratio unsimplified changes the formula.

Dot-and-cross: only outer shells and charges

Show only the outer shell of each ion, not the inner shells. After transfer the cation outer shell is empty ( $\text{Na}^+$ has 0 outer electrons, not 1) and the anion shows a full octet ( $\text{Cl}^-$ has 8, not 7). Enclose each ion in square brackets with its charge outside the top right, and use dots for one atom and crosses for the other.

W22 Paper 31 Q2(b); S22 Paper 31 Q5(e)(iv) — outer-shell electron counts drawn wrongly.

Exam tips

Formula scaffold: cross, simplify, check
Write both charges, cross-multiply each magnitude into the other subscript, simplify to the smallest ratio, then check subscript × charge sums to zero — for $\text{Al}_2\text{O}_3$ , $(2 \times 3+) + (3 \times 2-) = 0$ . Add brackets around any polyatomic ion used more than once.
Work out ion electrons from the charge
Start from the neutral atom, then remove electrons for a positive charge or add them for a negative charge. Na (2.8.1) loses 1 to give $\text{Na}^+$ 2.8; O (2.6) gains 2 to give $\text{O}^{2-}$ 2.8. Write 2.8, not 2.8.0 — an empty outer shell is left off.
Explain properties via forces and free ions
High melting point: strong electrostatic attractions between the ions need much energy to overcome. Conducts when molten or dissolved: the ions are free to move. Brittle: a blow aligns like charges, which repel. Always link the property to the lattice forces.

Revision notes