The ions you must know
Two rules generate almost every name: metals form positive ions, non-metals form negative ions; and the ending tells you about oxygen; -ide means the element alone (chloride, oxide, nitride), -ate means oxygen is in there too (sulfate, nitrate, carbonate). Where a metal can form more than one ion, the Roman numeral in brackets gives the charge: iron(II) is Fe2+, iron(III) is Fe3+.
To build a formula, start from one fact: a compound has no overall charge, so the positive and negative charges must cancel to zero. Take enough of each ion that the pluses and minuses balance — one Al3+ is +3, one O2− is −2, so you need two Al3+ (+6) and three O2− (−6) to reach zero: Al2O3. Crossing the charges over — the size of one ion's charge becoming the other's subscript — is just the quick way to land on those same numbers. Either way, if a polyatomic ion (NO3−, OH−, SO42−…) needs a subscript, bracket it first so the whole group multiplies.
Why metals lose electrons to form positive ions and non-metals gain them to form negative ones is C2 — Ionic bonding. If balancing the charges feels shaky, the ionic-formulae walk-through there builds MgCl2 one ion at a time until the + and − cancel.
| 1+ | 2+ | 3+ | 1− | 2− | 3− |
|---|---|---|---|---|---|
| Na+ K+ Li+ Ag+ NH4+ H+ | Mg2+ Ca2+ Ba2+ Sr2+ Zn2+ Cu2+ Fe2+ Pb2+ Ni2+ | Al3+ Fe3+ Cr3+ | Cl− Br− I− F− OH− NO3− HCO3− | O2− S2− SO42− CO32− | N3− PO43− |
Worked example — the formula of calcium nitrate
- Find the ions: calcium is Group 2 → Ca2+; nitrate is in the table → NO3−.
- Balance the charges to zero: one Ca2+ is +2, so you need two NO3− (2 × −1 = −2) to cancel it. (Crossing the charges over lands on the same 2 — it's the shortcut.)
- Bracket the polyatomic ion:
Ca(NO3)2
Plain text is fine: Na2CO3 reads as Na2CO3, brackets work as normal — Fe(OH)3 — and the preview beside the box shows exactly how your answer reads. Capital letters matter: CO is carbon monoxide, Co is cobalt.
Formula drill, level 1 — simple ionic
Twelve formulae, one at a time, shuffled each run. Type the formula, check it, and read the feedback on anything you miss — then retry the missed ones at the end until the deck is clean.
potassium bromide
- K+ and Br− are 1+ and 1− — they pair 1:1.
- Br− is only 1− — K+ and Br− pair 1:1.
calcium chloride
- Ca2+ needs two Cl− — cross the charges.
- Crossed the wrong way — one Ca2+ takes two Cl−.
sodium oxide
- Na+ and O2− — charges unequal; two Na+ needed.
magnesium sulfide
- Mg2+ and S2− both carry 2 — the charges cancel 1:1.
- Mg2+ and S2− both carry 2 — the charges cancel 1:1.
lithium nitride
- N3− needs three Li+.
- Crossed the wrong way — three Li+ balance one N3−.
aluminium chloride
- Al3+ needs three Cl−.
- Crossed the wrong way — one Al3+ takes three Cl−.
potassium sulfide
- K+ and S2− — charges unequal; two K+ needed.
- Upside down — S2− takes two K+.
barium iodide
- Ba2+ needs two I−.
- Crossed the wrong way — one Ba2+ takes two I−.
sodium fluoride
- Na+ and F− are 1+ and 1− — they pair 1:1.
- Na+ and F− are 1+ and 1− — they pair 1:1.
calcium oxide
- Ca2+ and O2− cancel exactly — 1:1.
- 2:2 isn't the simplest ratio — divide through: CaO.
strontium fluoride (deduce: Sr is in Group 2)
- Sr is in Group 2, so Sr2+ — two F− needed.
magnesium phosphide
- Mg2+ and P3− — cross the charges: Mg3P2.
- Crossed the wrong way — three Mg2+ balance two P3−.
Formula drill, level 2 — Roman numerals & polyatomic ions
Now the ions that carry their charge in the name — iron(III) means Fe3+, no exceptions — and the polyatomic groups that need brackets the moment they pick up a subscript.
iron(II) chloride
- (II) means Fe2+ — two Cl−, not three.
- Fe2+ needs two Cl−.
iron(III) sulfate
- That's iron(II) sulfate — the (III) means Fe3+.
- Crossed the wrong way: two Fe3+ balance three SO42−.
copper(II) nitrate
- Bracket the nitrate: Cu(NO3)2.
- Cu2+ needs two NO3−.
silver(I) oxide
- Ag+ and O2− — two Ag+ needed.
ammonium phosphate
- NH4+ is 1+ and PO43− is 3− — three ammonium ions, bracketed.
- Bracket the ammonium group: (NH4)3.
potassium carbonate
- K+ is 1+ and CO32− is 2− — two K+ needed.
- CO32− is 2− and K+ is only 1+ — it's two K+ to one carbonate.
calcium hydrogencarbonate
- Ca(HCO3)2 — brackets keep the hydrogencarbonate group together.
- HCO3− is 1− — Ca2+ needs two of them.
chromium(III) oxide
- (III) means Cr3+ — cross with O2−: Cr2O3.
- Crossed the wrong way — two Cr3+ balance three O2−.
zinc hydroxide
- Without brackets this reads as 1 O and 2 H — hydroxide is OH−: Zn(OH)2.
- Zn2+ needs two OH−.
lead(II) iodide
- Pb2+ needs two I−.
nickel(II) sulfate
- Ni2+ and SO42− cancel exactly — 1:1.
- That would need Ni4+ — (II) means Ni2+, so 1:1.
tin(IV) oxide
- (IV) means Sn4+ — two O2− needed.
- Simplify the ratio: SnO2.
Formula drill, level 3 — elements & molecules
Not everything is ionic. Remember the diatomic seven — H2, N2, O2, F2, Cl2, Br2, I2 — the elements that go around in pairs. Noble gases are single atoms; metals and giant structures are written as the bare symbol (Fe, C, SiO2 as a unit); and a handful of molecular compounds simply have to be known.
Have No Fear Of Ice Cold Beer — the first letters give the diatomic seven: Have (H2), No (N2), Fear (F2), Of (O2), Ice (I2), Cold (Cl2), Beer (Br2).
oxygen
- Oxygen the element exists as O2 molecules.
nitrogen
- One of the diatomic seven — N2.
iodine
- Iodine is in the diatomic seven — I2.
neon
- Noble gases don't pair up — full outer shell already.
iron
- Metals are written as the bare symbol — giant structures, not molecules.
ammonia
- NH4+ is the ammonium ion — ammonia the molecule is NH3.
carbon dioxide
- CO is carbon monoxide — dioxide means two oxygens.
- Watch the capitals: Co is cobalt — carbon dioxide is CO2.
sulfuric acid
- That's sulfurous acid — sulfuric is H2SO4.
- Two H+ balance the 2− sulfate.
silicon dioxide
- Dioxide — two oxygens: SiO2.
caesium oxide (deduce: Cs is in Group 1)
- Cs is Group 1 → Cs+; O2− needs two of them.
gallium oxide (deduce: Ga is in Group 3)
- Ga is Group 3 → Ga3+ — cross with O2−: Ga2O3.
- Crossed the wrong way — two Ga3+ balance three O2−.
rubidium sulfide (deduce)
- Rb is Group 1 → Rb+; S2− needs two of them.
Balancing equations
Balancing is bookkeeping: the same number of every atom on each side, changed only by the big numbers in front — never by editing a formula. Type the balancing numbers (a blank box counts as 1) and check; any correct multiple is accepted, and the feedback shows the simplest form.
Balancing and conservation of mass started at GCSE — see C3 — Balancing equations.
At A-Level, ½O2 is not just allowed — it's often preferred (e.g. in enthalpy equations “per mole of fuel”).
Try balancing this one with just ONE C2H6 — decimals allowed.
Equations from words
A-Level questions often describe a reaction in a sentence and expect the balanced equation back. The skill is spotting what the products must be — and what can't exist. Pick the correct balanced equation for each description.
Lithium reacts with water to form lithium hydroxide and hydrogen. Tick (✓) one box.
Complete combustion of ethene, C2H4. Tick (✓) one box.
Sulfuric acid neutralised by potassium hydroxide. Tick (✓) one box.
Thermal decomposition of copper(II) carbonate. Tick (✓) one box.
Magnesium burns in nitrogen to form magnesium nitride. Tick (✓) one box.
Stretch: ionic equations
In solution, soluble ionic compounds exist as separate ions — and often only some of them actually react. An ionic equation keeps the ions that change and crosses out the spectator ions that don't. At A-Level you'll also be expected to include state symbols: (s), (l), (g), (aq).
The one thing to get right is what splits and what stays whole. Only dissolved ionic compounds — the (aq) ones — break apart into separate ions. Everything else is written as a full formula: solids (s), liquids (l), gases (g), and covalent molecules like water. Split one of those by mistake and the equation is wrong.
Worked example — HCl + NaOH, spectators crossed out
- Write everything as ions:
H+(aq) + Cl−(aq) + Na+(aq) + OH−(aq) → Na+(aq) + Cl−(aq) + H2O(l)
- Cross out what appears unchanged on both sides: Na+ and Cl− are spectators.
- What's left is the ionic equation:
H+(aq) + OH−(aq) → H2O(l)
Worked example — magnesium displacing copper, spectators crossed out
- Start with the balanced equation:
Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s)
- Split only the (aq) compounds into ions — the two metals are solids, so they stay whole:
Mg(s) + Cu2+(aq) + SO42−(aq) → Mg2+(aq) + SO42−(aq) + Cu(s)
- Cancel the spectator: SO42− is identical on both sides, so it takes no part:
Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s)
You met all of this at GCSE — go back if it feels new. The full displacement walk-through, plus splitting an ionic equation into half equations, is C4 — Displacement; the universal neutralisation equation is C4 — pH & neutralisation; and the precipitate equations are C8 — Identifying anions.
Every strong acid–alkali neutralisation has the same ionic equation. Which? Tick (✓) one box.
Mixing barium chloride solution with sodium sulfate solution gives a white precipitate. Ionic equation? Tick (✓) one box.
Ionic equation for dilute acid + a carbonate solution? Tick (✓) one box.
How many moles of NaOH neutralise 1 mol of H2SO4?
How many moles of ammonia, NH3, react with 1 mol of HNO3?
Worked steps
- NH3 + H+ → NH4+ — one proton each
How did you do?
Work the drills above and your score appears here.
Run the drills again tomorrow — ten minutes a day beats two hours once. Next stop: moles, the currency of A-Level Chemistry.
That's fixable before September — I take on a small number of A-Level students each year →