Ionic Compounds: Formulas and Nomenclature

Ionic charges are determined from the group number of the element in the periodic table. Metals in group 2, like calcium, form cations with a 2+ charge. Nonmetals in group 17, like fluorine, form anions with a 1− charge. Group 16 elements gain two electrons to form anions with a 2− charge. Transition metals often exhibit variable charges that are not predictable by their location in the periodic table.

The crisscross method balances the overall charge within an ionic compound formula. The cationic subscript is numerically matched to the anionic charge and vice versa, generating a charge-neutral chemical formula. For example, in calcium fluoride, calcium's 2+ charge becomes fluorine's subscript, and fluorine's 1− charge becomes calcium's subscript, producing CaF2.

Roman numerals indicate the metal ion charge when a metal can form multiple cations with different charges. Transition metals like iron form two or more cations; iron forms both 2+ and 3+ ions. The compounds FeCl2 and FeCl3 are named iron(II) chloride and iron(III) chloride to distinguish them. Metals like sodium, which form only one cation type, do not require Roman numerals.

Oxyanions are named using suffixes and prefixes based on oxygen content. The most common oxyanion uses the suffix '–ate'. With one fewer oxygen, the suffix changes to '–ite'. One additional oxygen atom receives the prefix 'per–'. The oxyanion with one oxygen fewer than the '–ite' form gets the prefix 'hypo–'. For example, sodium bromate, bromite, perbromate, and hypobromite represent bromine oxyanions with varying oxygen counts.

Polyatomic ions are groups of bonded atoms acting as discrete units with an overall charge. In ionic formulas, parentheses indicate polyatomic ions that behave as a unit. For example, calcium phosphate contains the polyatomic ion PO43− and is written as Ca3(PO4)2. This formula shows three Ca2+ ions for every two PO43− groups, maintaining electrical neutrality with equal positive and negative charges.

Hydrated ionic compounds contain chemically-associated water molecules. The ionic part is named first, followed by a Greek numeral prefix indicating water molecule count and the suffix '–hydrate'. For example, CaCl2∙2H2O is calcium chloride dihydrate. The formula uses a centered dot, a coefficient for water molecules, and H2O to show the water association with the ionic compound.

Write the formula by treating polyatomic ions as discrete units and balancing charges. Determine the charges of the cation and polyatomic anion, then use subscripts to balance total positive and negative charges. For compounds containing polyatomic ions like CaSO4, the formula represents the empirical ratio. For sodium oxalate with Na+ and C2O42− ions in a 2:1 ratio, the formula is Na2C2O4, showing the relative numbers of constituent ions.