What are the rules for writing ionic compound formulae?

The rules for writing ionic compound formulae involve balancing the charges of the ions to achieve a neutral compound.

When writing the formula for an ionic compound, the first rule is to ensure that the total positive charge from the cations (positively charged ions) equals the total negative charge from the anions (negatively charged ions). This is because ionic compounds are neutral overall. For example, in sodium chloride (NaCl), the sodium ion (Na+) has a charge of +1 and the chloride ion (Cl-) has a charge of -1. The charges balance out to zero, making the compound neutral.

The second rule is to use the simplest ratio of ions. This is known as the empirical formula. For instance, in magnesium chloride (MgCl2), the magnesium ion (Mg2+) has a charge of +2 and each chloride ion (Cl-) has a charge of -1. Two chloride ions are needed to balance the +2 charge of the magnesium ion, hence the formula is MgCl2, not Mg2Cl2.

The third rule is to use parentheses to indicate the number of polyatomic ions in the formula. Polyatomic ions are groups of atoms that have an overall charge. For example, in calcium hydroxide (Ca(OH)2), the calcium ion (Ca2+) has a charge of +2 and the hydroxide ion (OH-) has a charge of -1. Two hydroxide ions are needed to balance the +2 charge of the calcium ion, hence the formula is Ca(OH)2, not CaO2H2.

Lastly, when writing the formula, the cation is always written first, followed by the anion. This is a convention in chemistry that helps to clearly identify the components of the compound. For example, in sodium chloride, sodium (the cation) is written first, followed by chloride (the anion).