Formula of Ionic Compounds

Factoids

• A chemical compound is always electrically neutral.

• The positive and negative valencies of the ions/radicals in a compound are equal and balanced.

Steps

• Write the symbol of the ions of the compound.

• Cation (positive ion) is written on the left hand side and the anion (negative ion) is written on the right hand side.

• Put the valency number of the radical or ion below the symbol of the element.

• Alternatively cross the positive and negative charges on ions to give subscripts.

Example:

Calcium ChlorideCa²⁺ Cl^1-Valencies 2 1

Formula CaCl₂

Ensure that the charges are balanced.

1(Ca²⁺) = +2 2(Cl^-) = -2

• Radicals need to be enclosed within brackets if their number exceeds one.

Example:

Aluminium SulphateAl³⁺ SO₄^2-Valency 3 2

2(Al³⁺) = + 6 3(SO₄^2-) = -6

Al₂ (SO₄)₃.