All courses › Chemistry 1 › Formula unit and ion charge

Formula unit and ion charge

The formula of an ionic compound is chosen so that the total charge is zero. An ion of charge +2 must be balanced by two ions of charge -1, or by one ion of charge -2. Magnesium is in group 2 and forms Mg2+. Chloride has charge -1, so the formula is MgCl2. You find the molar mass of a formula unit by adding the atomic masses. For NaCl the masses 22.99 for sodium and 35.45 for chlorine are used.

\mathrm{Mg}^{2+} + 2\,\mathrm{Cl}^- \rightarrow \mathrm{MgCl}_2

the charges cancel

M(\mathrm{NaCl}) = M(\mathrm{Na}) + M(\mathrm{Cl})

molar mass

Symbols

$Mg$	charge +2 in the group
$Na$	22.99	g/mol
$Cl$	35.45	g/mol

Example

The charge in MgCl2: $2 + 2 \cdot (-1) = 0$ .

NaCl: $22.99 + 35.45 = 58.44$ g/mol.

Write the positive ion first. Cross the charges over if you need the count of each ion.

Practise chemical bonds for free →

← Covalent bonding · Molar mass →

Part of Chemistry 1: Chemical bonds.