Writing Ionic Formulae

Key Concepts

Positively charged ions are called cations
Negatively charged ions are called anions
The formula of an ionic compound represents the simplest whole number ratio of ions present.
The net charge on an ionic compound is zero, so the sum of the positive charges equals the sum of the negative charges.
A subscript number written to the right of an element's symbol tells us how many of those ions are present in the formula.
for example: potassium sulfide has the formula K₂S, 2 potassium ions (K⁺) and 1 sulfide ion (S^2-) are present.
If no number is given, then only one of that ion is present.
for example: sodium hydride has the formula NaH, 1 sodium ion (Na⁺) and 1 hydride ion (H⁺) are present.
If more than one polyatomic ion (ion having more that one element in its formula) is present, round brackets enclose the formula of the polyatomic ion and a subscript number written to the right of the final bracket tells us how many of that polyatomic ion are present in the formula.
for example, the nitrate ion is a polyatomic ion, NO₃^-, it is an ion containing nitrogen and oxygen atoms. Calcium nitrate has the formula Ca(NO₃)₂ and it contains 1 calcium ion (Ca²⁺) and 2 nitrate ions (NO₃^-).

Type of Ion	Charge	Examples
CATIONS	+1	Group I ions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) Ag⁺, H⁺, NH₄⁺[amminium], Cu⁺[copper (I)], Hg₂²⁺[mercury(I)]

	+2	Group II ions (Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) Zn²⁺, Cu²⁺[copper (II)], Fe²⁺[iron (II)], Pb²⁺[lead (II)], Sn²⁺[tin (II)], Hg²⁺[mercury (II)]

	+3	Al³⁺, Fe³⁺[iron (III)]

	+4	Pb⁴⁺[lead(IV)], Sn⁴⁺[tin(IV)]

ANIONS	-1	Group VII ions (F^-[fluoride], Cl^-[chloride], Br^-[bromide], I^-[iodide]) H^-[hydride], OH^-[hydroxide], NO₃^-[nitrate], NO₂^-[nitrite]

	-2	Group VI ions (O^2-[oxide], S^2-[sulfide]) SO₄^2-[sulfate], SO₃^2-[sulfite], CO₃^2-[carbonate]

	-3	N^3-[nitride], P^3-[phosphide], PO₄^3-[phosphate]

magnesium oxide

CATION: magnesium (Group II) charge is +2: Mg²⁺
ANION: oxide ion (Group VI) charge is -2: O^2-
1 Mg²⁺ cation and 1 O^2- anion combine to give a compound with zero net charge,
+2 + -2 = 0
Formula of magnesium oxide is MgO

ammonium hydroxide

CATION: ammonium ion, charge is +1: NH₄⁺
ANION: hydroxide ion, charge is -1: OH^-
1 NH₄⁺ cation and 1 OH^- anion combine to give a compound with zero net charge,
+1 + -1 =0
Formula of ammonium hydroxide is NH₄OH

aluminium chloride

CATION: aluminium (Group III), charge is +3: Al³⁺
ANION: chloride (Group VII), charge is -1: Cl^-
1 Al³⁺ cation and 3 Cl^- anions combine to give a compound with zero net charge:
+3 + (3 x -1) = 0
Formula of aluminium chloride is AlCl₃

sodium oxide

CATION: sodium (Group I), charge is +1: Na⁺
ANION: oxide (Group VI), charge is -2: O^2-
2 Na⁺ cations and 1 O^2- anion combine to give a compound with zero net charge:
(2 x -1) + -2 = 0
Formula of sodium oxide is Na₂O

ammonium sulfate

CATION: ammonium, charge is +1: NH₄⁺
ANION: sulfate, charge is -2: SO₄^2-
2 NH₄⁺ cations and 1 SO₄^2- anion combine to give a compound with zero net charge:
(2 x -1) + -2 = 0
Formula of ammonium sufate is (NH₄)₂SO₄

barium hydroxide

CATION: barium (Group II), charge is +2: Ba²⁺
ANION: hydroxide, charge is -1: OH^-
1 Ba²⁺ cation and 2 OH^- anions combine to give a compound with zero net charge:
+2 + (2 x -1) = 0
Formula of barium hydroxide is Ba(OH)₂