Names and formulae of ionic compounds

To deduce the formulae of , the formulae of their can be used.

The table shows the names and formulae of some common ions.

Cations	Anions
Lithium, Li⁺	Chloride, Cl^-
Sodium, Na⁺	Bromide, Br^-
Potassium, K⁺	Iodide, I^-
Silver(I), Ag⁺	Oxide, O^2-
Barium, Ba²⁺
Calcium, Ca²⁺
Copper(II), Cu²⁺
Iron(II), Fe²⁺
Lead(II), Pb²⁺
Magnesium, Mg²⁺
Zinc, Zn²⁺
Aluminium, Al³⁺
Iron(III), Fe³⁺

Cations	Lithium, Li⁺
Anions	Chloride, Cl^-

Cations	Sodium, Na⁺
Anions	Bromide, Br^-

Cations	Potassium, K⁺
Anions	Iodide, I^-

Cations	Silver(I), Ag⁺
Anions	Oxide, O^2-

Cations	Barium, Ba²⁺

Cations	Calcium, Ca²⁺

Cations	Copper(II), Cu²⁺

Cations	Iron(II), Fe²⁺

Cations	Lead(II), Pb²⁺

Cations	Magnesium, Mg²⁺

Cations	Zinc, Zn²⁺

Cations	Aluminium, Al³⁺

Cations	Iron(III), Fe³⁺

Working out a formula

The formula for an ionic compound must contain the same number of positive and negative so that the charges are balanced and it is neutral overall. Here are some examples.

Example 1

Sodium chloride contains Na⁺ and Cl^- ions:

this is one positive charge and one negative charge
the charges are balanced
so the formula is NaCl

Example 2

Magnesium oxide contains Mg²⁺ and O^2- ions:

this is two positive charges and two negative charges
the charges are balanced
so the formula is MgO

Example 3

Aluminium oxide contains Al³⁺ and O^2- ions:

this is three positive charges and two negative charges
to balance, we need two Al³⁺ ions and three O^2- ions
so the formula is Al₂O₃

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Ionic compounds - EdexcelNames and formulae of ionic compounds

Names and formulae of ionic compounds

Working out a formula

Example 1

Example 2

Example 3

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