Amount of Substance

Formulae of Ionic Compounds

Formulae of Ionic Compounds

In an ion, the number of electrons is not equal to the number of protons. The charge of an ion, therefore, is the difference between its protons and electrons.

  • Positively charged ions are called cations.
  • Negatively charged ions are called anions.

A compound composed of ions is known as a salt. They are always electrically neutral. They do not form molecules and therefore have no molecular formulae. Instead, they are written as unit formulae.

The unit formulae of an ionic compound represents the simplest whole number ratio in which the ions in a compound exist. This is dependent on the ion charges involved. The tables below show some of the most important ions.

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Cations

Charge

Formula

Name

+1

+1

+1

+1

+1

+1

+2

+2

+2

+2

+2

+2

+2

+3

+3

+3

Na+

K+

Ag+

H+

NH4+

Cu+

Mg2+

Ca2+

Fe2+

Zn2+

Pb2+

Cu2+

Ni2+

Al3+

Cr3+

Fe3+

Sodium

Potassium

Silver

Hydrogen

Ammonium

Copper(I)

Magnesium

Calcium

Iron(II)

Zinc

Lead(II)

Copper(II)

Nickel(II)

Aluminium

Chromium(III)

Iron(III)

Atoms that can form more than one stable cation must include the charge in brackets after the metal’s name.

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Anions

Charge Formula Name

-1

-2

-2

-1

-1

OH-

SO42-

CO32-

NO3-

HCO3-

Hydroxide

Sulphate

Carbonate

Nitrate

Hydrogencarbonate