Two elements, one metal and one non-metal, and a formula you are expected to produce from nothing but the periodic table. There is no memorising involved here — just charge balance, applied carefully and written down properly.
📘 What you need to know
A binary ionic compound contains ions of exactly two elements: a metal cation and a non-metal anion.
Ionic bonding is the electrostatic attraction between oppositely charged ions. Learn that wording.
Naming: metal first, unchanged; then the non-metal with the ending changed to -ide.
For transition metals, add a Roman numeral giving the charge on the metal ion.
The compound must be electrically neutral, so total positive charge = total negative charge.
Polyatomic ions are charged groups of covalently bonded atoms, such as SO42− and NH4+.
If you need more than one polyatomic ion, put it in brackets: Ca(NO3)2.
What ionic bonding is
Students often describe ionic bonding as “the transfer of electrons”. That describes how the ions are made, not what the bond is. The bond is what happens afterwards.
Definition
Ionic bonding is the electrostatic attraction between oppositely charged ions.
Note the size change on the right: the sodium ion has shrunk and the chloride ion has grown, which is exactly what you would expect from the previous page.
A real ionic bond is not a private arrangement between one Na+ and one Cl−. Every ion attracts every oppositely charged ion around it, in all directions. This diagram shows one pair only because it is easier to draw, not because that is how the solid works.
Naming binary ionic compounds
The rule is short. Metal first, exactly as it appears on the periodic table. Non-metal second, with its ending replaced by -ide.
Non-metal
Ion name
Ion formula
Charge
chlorine
chloride
Cl−
1−
bromine
bromide
Br−
1−
oxygen
oxide
O2−
2−
sulfur
sulfide
S2−
2−
nitrogen
nitride
N3−
3−
phosphorus
phosphide
P3−
3−
hydrogen
hydride
H−
1−
Hydrogen is the odd one. With a reactive metal it behaves as a non-metal and forms H−, the hydride ion — so sodium and hydrogen give sodium hydride, NaH. With non-metals it does the opposite and forms H+.
Where the metal is a transition element, the name has to say which ion it is. Iron(II) sulfide is FeS; iron(III) sulfide is Fe2S3. Without the numeral the name is ambiguous, and an ambiguous name will not earn the mark.
Polyatomic ions
Some ions are not single atoms but small covalently bonded groups carrying an overall charge. They travel through reactions as a unit, so treat each one as a single indivisible ion with a single charge.
Name
Formula
Charge
Seen in
ammonium
NH4+
1+
ammonium sulfate fertiliser
hydroxide
OH−
1−
sodium hydroxide
nitrate
NO3−
1−
potassium nitrate
hydrogencarbonate
HCO3−
1−
baking soda
carbonate
CO32−
2−
limestone, chalk
sulfate
SO42−
2−
copper(II) sulfate
phosphate
PO43−
3−
bone mineral, fertiliser
Notice these end in -ate, not -ide. That ending is a signal: an -ate ion almost always contains oxygen. So if a compound is called sodium sulfate rather than sodium sulfide, you know immediately that oxygen is in there and the ion is SO42−.
Working out the formula
Ionic compounds have no overall charge. That single fact gives you the formula every time.
The rule
total positive charge = total negative charge
Some teachers show a criss-cross shortcut where the charges swap over as subscripts. It works, but it can leave you with Mg2O2 instead of MgO, so always simplify at the end.
🧩 Building an ionic formula
Write the two ions with their charges. Get these from the group number, the Roman numeral, or the polyatomic ion table.
Find the lowest common multiple of the two charge sizes. For 3+ and 2− that is 6.
Work out how many of each ion you need to reach it: 2 × 3+ and 3 × 2−.
Write the cation first, then the anion, with those numbers as subscripts. Leave out any subscript of 1.
Simplify if both subscripts share a factor. Mg2O2 must be written MgO.
Bracket any polyatomic ion that appears more than once, then put the subscript outside the bracket.
Worked examples
WORKED EXAMPLE
Naming the products
Give the name of the binary ionic compound formed in each reaction: potassium with bromine, magnesium with phosphorus, and barium with oxygen.
Rule: metal unchanged, non-metal ends in -idePotassium + bromineBromine becomes bromide.potassium bromideMagnesium + phosphorusPhosphorus becomes phosphide.magnesium phosphideBarium + oxygenOxygen becomes oxide.barium oxideno Roman numerals needed — none of these metals is a transition element
WORKED EXAMPLE
A formula that needs both subscripts
Deduce the formula of magnesium nitride.
Step 1: write the ionsMagnesium is Group 2, so Mg²⁺. Nitrogen is Group 15, so N³⁻.Step 2: lowest common multiple of 2 and 3LCM = 6Step 3: how many of each?3 × 2+ = 6+ so 3 Mg²⁺2 × 3− = 6− so 2 N³⁻Mg₃N₂the 3 goes on the magnesium even though its charge is 2 — that is the whole point of the method
WORKED EXAMPLE
A formula with a polyatomic ion
Deduce the formula of aluminium sulfate.
Step 1: write the ionsAluminium is Group 13, so Al³⁺. Sulfate is SO₄²⁻.Step 2: balance to the LCM of 3 and 2LCM = 62 Al³⁺ gives 6+ 3 SO₄²⁻ gives 6−Step 3: bracket the polyatomic ionThree sulfates are needed, so SO₄ goes inside brackets with the 3 outside.Al₂(SO₄)₃writing Al₂SO₄₃ would mean something completely different
WORKED EXAMPLE
Working backwards to a name
Deduce the names of Cu2O and CuO.
Step 1: the oxide charge is fixedOxygen is Group 16, so every oxide ion is O²⁻.Step 2: Cu₂O — one oxide means 2− to balance2 copper ions share 2+, so each is 1+copper(I) oxideStep 3: CuO — one copper balances one O²⁻that single copper must carry 2+copper(II) oxidethe numeral and the subscript are different numbers here — a classic trap
💡 Exam tip
Write the two ions with charges before you attempt the formula. This is usually a mark in itself and it makes the balance obvious.
Learn the definition of ionic bonding word for word: the electrostatic attraction between oppositely charged ions.
Never leave a subscript of 1 in a formula. NaCl, not Na1Cl1.
Simplify the ratio. The formula of an ionic compound is empirical, so Ca2O2 is wrong even though the charges balance.
Brackets go around a polyatomic ion only when you need more than one of it. Ca(OH)2 takes brackets; NaOH does not.
The seven polyatomic ions are worth memorising outright — they appear across the whole course, not just here.
⚠ Common mix-up
Defining ionic bonding as electron transfer. The transfer makes the ions; the bond is the attraction that follows.
Forgetting the brackets. Mg(NO3)2 has two nitrates. MgNO32 is meaningless.
Confusing -ide with -ate. Sulfide is S2−; sulfate is SO42−. The oxygen changes everything.
Leaving out the Roman numeral for a transition metal, or reading it as a number of atoms.
Failing to simplify. The criss-cross shortcut gives Mg2O2; the answer is MgO.
Putting the non-metal first. The cation always comes first, in both the name and the formula.
Up next: Ionic Lattice Structures — why there is no such thing as a molecule of sodium chloride, and how the giant lattice explains every physical property ionic compounds have.
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