Using equations to represent chemical reactionsWriting formulae of ionic compounds
All substances are described by their formulae, which are used to write balanced chemical equations. Writing the formula for an ionic compound requires knowledge of the charges on its ions.
The formulaA combination of symbols that indicates the chemical composition of a substance. of an ionic compoundAn ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). shows the relative numbers of the ionElectrically charged particle, formed when an atom or molecule gains or loses electrons. it contains. For example:
the formula for sodium chloride is NaCl - it shows that for every Na+ ion there is one Cl- ion
the formula for magnesium chloride is MgCl2 - it shows that for every Mg2+ ion there are two Cl- ions
Working out an ionic formula
The formula for an ionic compound must contain the same number of positive and negative charge (electrical)An imbalance of electrons and protons in a material. An excess of electrons results in negative charge, a deficit of electrons results in positive charge.. This is so that the charges are balanced and the compound 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 (add up to zero)
so the formula is NaCl
Example 2
Magnesium oxide contains Mg2+ and O2- 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 O2- ions:
this is three positive charges and two negative charges
to balance we need, two Al3+ ions and three O2- ions