Formulae of Ionic Compounds

Ionic Compounds Formulae

Ionic compounds are composed of positively charged ions (cations) and negatively charged ions (anions).
The overall charge of an ionic compound is always neutral; therefore, the total positive charge from cations must equal the total negative charge from anions.
The formula of an ionic compound is determined by the charges of its constituent ions. For example, in sodium chloride (NaCl), sodium (Na) carries a +1 charge and chloride (Cl) carries a -1 charge, thus one Na ion combines with one Cl ion to give a neutral compound.
Transition metals: many transition metals can form ions with various charges, and the charge is denoted in Roman numerals after the metal name, e.g., iron(III) chloride (FeCl3), where the iron(Fe) forms a +3 charge ion.
Polyatomic ions: these are groups of atoms that stay together and carry an overall charge, such as nitrate (NO3-) or sulphate (SO4^2-). The formula of a compound containing a polyatomic ion must take into account the charge of the entire ion.
When writing compound formulae where subscripts are needed (numbers in the formula that show the quantity of a given element or compound), they should be written as lower numbers after the symbol (e.g., CaCl2).
When determining the formula of a compound containing polyatomic ions and simple ions, you require a number of polyatomic ions to balance the charge of the simple ions. These are shown by parentheses, and the subscript applies to all atoms inside the parentheses (e.g., (NH4)2SO4).

Balancing Charges in Ionic Compounds

Remember, the overall charge in an ionic compound must balance to zero. This means the sum of the charges from all cations equals the sum of the charges from all anions.
If there is an imbalance between positively charged cations and negatively charged anions, more ions are needed on one side to achieve neutrality.
This can be solved by adding more ions (represented as subscripts in the formula) until the charges balance out.
For instance, for calcium chloride (CaCl2), the calcium ion (Ca2+) has a +2 charge, whereas the chloride ion (Cl-) has a -1 charge. Therefore, two chloride ions are required to balance the charge of one calcium ion.