Structure and bonding · GCSE Chemistry
Ions and formulae
GCSE Chemistry revision on ions and formulae: charges on Na⁺, Mg²⁺, O²⁻, SO₄²⁻, balancing charges in salts, and writing formulae with state symbols.
The formula is the simplest ratio that makes the total positive charge equal the total negative charge. Swap the numbers, not the signs: Mg²⁺ and Cl⁻ make MgCl₂.
The important bits
What you need to know
- 1
A positive ion (cation) is formed when a metal atom loses electrons. Na → Na⁺ + e⁻; Mg → Mg²⁺ + 2e⁻; Al → Al³⁺ + 3e⁻.
- 2
A negative ion (anion) is formed when a non-metal atom gains electrons. Cl + e⁻ → Cl⁻; O + 2e⁻ → O²⁻; N + 3e⁻ → N³⁻.
- 3
Group 1 ions are 1+, Group 2 are 2+, Group 3 (Al) is 3+, Group 6 are 2−, Group 7 are 1−. That follows from the electrons needed for a full outer shell.
- 4
You must learn common compound ions: OH⁻ hydroxide, SO₄²⁻ sulfate, CO₃²⁻ carbonate, NO₃⁻ nitrate, NH₄⁺ ammonium, HCO₃⁻ hydrogencarbonate.
- 5
Ionic formulae balance charge. Ca²⁺ and Cl⁻ give CaCl₂; Al³⁺ and O²⁻ give Al₂O₃; Na⁺ and CO₃²⁻ give Na₂CO₃; Mg²⁺ and SO₄²⁻ give MgSO₄.
- 6
Brackets are needed when more than one compound ion is required: Ca(OH)₂, Al(NO₃)₃, (NH₄)₂SO₄. The subscript multiplies the whole ion.
- 7
State symbols complete an equation: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution. NaCl(s) is the lattice; NaCl(aq) is separated ions in water.
- 8
The empirical formula of an ionic compound is the simplest whole-number ratio of ions, not a molecule. There is no “NaCl molecule” in the lattice.
Quotations worth analysing
Short evidence. Real method.
“Na⁺, Mg²⁺, Al³⁺, Cl⁻, O²⁻, OH⁻, SO₄²⁻, CO₃²⁻, NO₃⁻, NH₄⁺”
Metals from Groups 1–3 and the listed compound ions appear in almost every salt, electrolysis and titration paper. Learn the charge with the name.
“Al₂O₃ MgCl₂ Ca(OH)₂ (NH₄)₂SO₄”
Cross the charge numbers (ignore signs) and simplify if you can. Al³⁺ with O²⁻ is 2 aluminium and 3 oxide. Mg²⁺ with SO₄²⁻ is already 2+ and 2−, so MgSO₄.
“H⁺(aq) + OH⁻(aq) → H₂O(l)”
State symbols matter: aqueous ions become liquid water. Spectator ions such as Na⁺ and Cl⁻ are left out of the ionic equation.
Go deeper
Balance charge, then check the brackets
Write the two ions with their charges. The total positive must equal the total negative. For aluminium sulfate that is Al³⁺ and SO₄²⁻, so two Al³⁺ (6+) and three SO₄²⁻ (6−) give Al₂(SO₄)₃. Without brackets you would write Al₂SO₄₃, which is meaningless. Magnesium nitrate is Mg(NO₃)₂ because two nitrate ions are needed to cancel Mg²⁺. Students forget that nitrate is 1−, or they treat SO₄ as S and O₄ separately. Name the ion as a unit, then multiply the unit. Ammonium sulfate is (NH₄)₂SO₄ for the same reason: two NH₄⁺ cancel one SO₄²⁻.
Go deeper
Ions keep the nucleus; only the electron count changes
Fe²⁺ and Fe³⁺ are still iron: 26 protons. Fe²⁺ has lost two electrons; Fe³⁺ has lost three. That is why transition-metal questions (Triple) ask you to state the charge. In Combined Science you still meet Zn²⁺, Cu²⁺ and Fe²⁺/Fe³⁺ in salts and in the reactivity series. Oxide is O²⁻, not O₂⁻. Peroxide is beyond GCSE. Hydroxide is OH⁻, a compound ion, not a mixture of oxygen and hydrogen gases. If you can write the ion, you can name the salt: copper(II) sulfate is CuSO₄ because copper is 2+ and sulfate is 2−.
Go deeper
State symbols tell you whether ions can move
NaCl(s) does not conduct: ions are locked in the giant lattice. NaCl(l) and NaCl(aq) do conduct, because ions are free to move. That sentence links this topic to electrolysis and to bonding. In equations, hydrogen from a metal plus acid is H₂(g), carbon dioxide from a carbonate is CO₂(g), and water is H₂O(l) unless it is steam, H₂O(g). Marks are lost for writing HCl(g) when the acid is a solution: hydrochloric acid is HCl(aq). Use (aq) for anything dissolved in water. A missing state symbol is a cheap way to drop a mark on an otherwise correct equation.
See the idea in action
Write the formula of aluminium sulfate. Ions: Al³⁺ and SO₄²⁻. Lowest common multiple of the charges is 6, so two Al³⁺ and three SO₄²⁻. Formula Al₂(SO₄)₃. Check: 2 × 3+ = 6+ and 3 × 2− = 6−. For calcium nitrate: Ca²⁺ and NO₃⁻ give Ca(NO₃)₂. Equation with state symbols: 2Al(s) + 3Cl₂(g) → 2AlCl₃(s). Aluminium chloride is AlCl₃ because Al³⁺ needs three Cl⁻.
Exam technique
Turn knowledge into marks
Jot the ions and charges in the margin before you write the formula. If a compound ion appears more than once, use brackets. Finish symbol equations with (s), (l), (g) or (aq).
Common mistakes
Do not give these marks away
- 01
Writing MgCl instead of MgCl₂, or NaSO₄ instead of Na₂SO₄.
- 02
Omitting brackets in Ca(OH)₂ or Al(NO₃)₃, or giving sulfate the charge 1−.
- 03
Using (g) for hydrochloric acid, or writing NaCl as a molecule rather than a giant ionic lattice.
What is the correct formula of magnesium hydroxide?
AMgOH
BMg(OH)₂
CMg₂OH
DMgOH₂
Show the answer
Mg(OH)₂. Magnesium forms Mg²⁺ and hydroxide is OH⁻. Two hydroxide ions are needed to cancel the 2+ charge, and brackets show that both OH units are attached.
Quick questions
If this is the bit you searched
How do you work out the formula of an ionic compound GCSE?
Write the ions with charges, then find the simplest ratio that makes total positive charge equal total negative charge. Use brackets around compound ions if you need more than one.
What is the charge on sulfate, carbonate and nitrate?
Sulfate is SO₄²⁻, carbonate is CO₃²⁻ and nitrate is NO₃⁻. Hydroxide is OH⁻ and ammonium is NH₄⁺.
Why is the formula of aluminium oxide Al2O3?
Al³⁺ and O²⁻ need two aluminium ions (6+) and three oxide ions (6−) so the charges cancel. The simplest ratio is Al₂O₃.
When do you use brackets in chemical formulae?
When a compound ion such as OH⁻, NO₃⁻ or SO₄²⁻ appears more than once, for example Ca(OH)₂ or Al(NO₃)₃. The subscript multiplies the whole ion.