A Chemical Formula consists of:

· The chemical symbol of the element

· The number of atoms (numbers after the chemical symbol. If there is no subscript, it means there is only one atom of the element)

e.g. N₂ : 1 type of element, nitrogen. Total no of atoms: 2

Why does an atom become an ion?

è The reason is because these atoms want to have a full and balanced valence shell which consists of 8 electrons.

è Hence, these atoms will either lose or gain electrons to become either an anion or a cation respectively.

è Ions are an ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons.

v Atoms that belong to groups 1, 2, 3 and 4 usually lose electrons to achieve a full valence shell.

v These atoms belong to the metallic group.

v Atoms that belong to groups 5, 6 and 7 usually gain electrons to achieve a full valence shell.

v These atoms belong to the non-metallic group.

General rules to writing chemical formulae:

v Always write the metallic (positive ion/cation) part first.

v The overall charge of the compound is always 0. The total + and – charges are equal.

v The number of each ion required is written as a subscript. Eg. MgCl₂

v If the number of polyatomic ion required is more than one, brackets are required. Eg Mg(OH)₂