Formulae of Ionic Compounds

Ionic compounds are formed when a metal and a non-metal react together. The metal loses electrons to form a positive ion (cation) and the non-metal gains electrons to form a negative ion (anion).
The formula of an ionic compound reflects the balance of charges in the compound. The total positive charge from the metal ions must equal the total negative charge from the non-metal ions. This ensures that the compound is overall electrically neutral.
The formula for sodium chloride, NaCl, for example, reflects the fact that each sodium ion (Na+) balances out each chloride ion (Cl-) in the compound.
Polyatomic ions are groups of atoms that have an overall charge. For example, the hydroxide ion (OH-) is a negatively charged ion composed of one oxygen atom and one hydrogen atom.
When writing the formula for an ionic compound involving polyatomic ions, the polyatomic ion is treated as a single unit. For example, the formula for calcium hydroxide is Ca(OH)2 not CaO2H2. This indicates that there is one calcium ion for every two hydroxide ions in the compound.
When it comes to compounds with transition metals that can form more than one type of ion, Roman numerals are used to indicate the charge of the ion. For example, iron(II) sulphate is FeSO4, containing Fe2+ ions, and iron(III) sulphate is Fe2(SO4)3, containing Fe3+ ions.
Lastly, the formula of an ionic compound does not tell us about its structure. Ionic compounds are usually arranged in a giant ionic lattice structure. This is a regular, repeating arrangement of ions and gives ionic compounds their distinctive properties such as high melting points and the ability to conduct electricity when molten or dissolved in water.