Ionic bonding — AQA A-Level Chemistry
Test yourself on Ionic bonding with AQA A-Level practice questions.
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Ionic bonding explained
Ionic bonding forms when electrons transfer from a metal to a non-metal, leaving oppositely charged ions.
Read the full explanation
The bond itself is the electrostatic attraction between those opposite charges, and it acts in every direction, so the ions build a giant lattice rather than a molecule. In sodium chloride each sodium ion is surrounded by six chloride ions and each chloride by six sodium ions. Lattice strength rises as the charge on the ions rises and as the ionic radii fall, because the attraction depends on charge density; magnesium oxide therefore melts far above sodium chloride. Melting or dissolving frees the ions to move, which is why ionic compounds conduct only when molten or in solution, never as solids.
The formulas of compound ions, eg sulfate, hydroxide, nitrate, carbonate and ammonium.
Compound ions are groups of covalently bonded atoms carrying an overall charge, and they must be learned as complete units: sulfate SO₄²⁻, hydroxide OH⁻, nitrate NO₃⁻, carbonate CO₃²⁻ and ammonium NH₄⁺. Ammonium is the only common positive one. In a formula the charge is balanced against the counter-ion, and if more than one compound ion is needed the whole ion is bracketed before the subscript, giving Ca(OH)₂, Al₂(SO₄)₃ and (NH₄)₂CO₃. The bracket matters for both the formula and its relative formula mass, since the subscript multiplies every atom inside it. Related ions differ only in oxygen count and charge, so learn hydrogencarbonate HCO₃⁻ alongside carbonate rather than deriving it.
Students should be able to: predict the charge on a simple ion using the position of the element in the Periodic Table construct formulas for ionic compounds.
The group number predicts the ion. Group 1 loses one electron to give 1⁺, Group 2 gives 2⁺, Group 3 gives 3⁺; Group 5 gains three for 3⁻, Group 6 gains two for 2⁻ and Group 7 gains one for 1⁻. In each case the ion reaches the electron configuration of the nearest noble gas. Transition metals vary, so their charge is stated by the Roman numeral in the name, as in iron(III). To build a formula, write the two ions with their charges and add whichever numbers make the total positive charge equal the total negative charge, then reduce to the simplest ratio: magnesium oxide is MgO, not Mg₂O₂, and aluminium oxide is Al₂O₃.
Your focus
- Explain the difference in melting point between magnesium oxide and sodium chloride using ionic charge and radius.
- Describe the arrangement of ions in the sodium chloride lattice and the direction in which the attraction acts.
- Justify why an ionic solid does not conduct electricity but its molten form does.
Show all 9 objectives
- Write the formula, including charge, for sulfate, hydroxide, nitrate, carbonate and ammonium from memory.
- Construct the formula of a salt that requires more than one compound ion, using brackets correctly.
- Calculate the relative formula mass of a compound whose formula contains a bracketed compound ion.
- Predict the charge on the ion formed by a main-group element from its position in the Periodic Table.
- Construct the formula of an ionic compound by balancing total positive against total negative charge.
- Deduce the charge on a transition metal ion from the Roman numeral in the compound's name.
Ionic bonding exam tips
Marking Points
- one mark for electrostatic attraction, not simply attraction
- one mark for the attraction being between oppositely charged ions
- one mark for the ions arranged in a giant lattice with the attraction acting in all directions
- one mark for linking a higher ionic charge or a smaller ionic radius to a stronger lattice and a higher melting point
- one mark for the correct atoms in the compound ion
- one mark for the correct charge on that ion
- one mark for brackets used whenever more than one compound ion appears in a formula
- one mark for a formula with no overall charge once the ions are combined
- one mark for the charge on each ion deduced from its group in the Periodic Table
- one mark for the total positive charge balanced against the total negative charge
- one mark for the formula reduced to its simplest whole-number ratio
- one mark for brackets around a compound ion that appears more than once
Examiner Tips
- 💡Use the phrase electrostatic attraction between oppositely charged ions in every definition; it is the marking phrase.
- 💡When comparing two lattices, compare both charge and radius before drawing a conclusion.
- 💡Explain conductivity by whether the ions are free to move, not by whether electrons are free.
- 💡Memorise the five compound ions with their charges as a single list; they appear across the whole specification.
- 💡Bracket first, then add the subscript, so no atom inside is missed.
- 💡Check the finished formula sums to zero charge before moving on.
- 💡Write the two ions with charges above them before combining; the cross-over then follows automatically.
- 💡Always reduce the finished subscripts by any common factor.
- 💡For a transition metal, read the Roman numeral in the name as the charge on the ion.
Common Mistakes
- describing ionic bonding as the sharing of electrons
- writing attraction between charges without the word electrostatic
- calling sodium chloride a molecule or referring to forces between its molecules
- claiming a solid ionic compound conducts electricity
- writing sulfate as SO₄⁻ or nitrate as NO₃²⁻
- omitting brackets so calcium hydroxide is written as CaOH₂
- giving ammonium as NH₃⁺ or leaving the charge off altogether
- assuming carbonate and hydrogencarbonate carry the same charge
- giving oxygen as O⁻ instead of O²⁻
- leaving magnesium oxide as Mg₂O₂ rather than reducing it
- ignoring the Roman numeral so iron(III) chloride is written as FeCl₂
- writing a finished formula that still carries an overall charge