Ionic Compounds: Formulas and Nomenclature

Ionic compounds form ionic bonds between positively charged metal ions, or cations, and negatively charged nonmetal ions, or anions. These compounds are represented by chemical formulas.

Consider calcium fluoride. First, specify the ions of each element and their respective ionic charges. Start with the cations followed by anions.

Ionic charges can be determined from the group number of the element in the periodic table. Calcium from group 2 forms cations with an ionic-charge of 2+. Conversely, fluorine from group 17 forms anions with an ionic-charge of 1−.

Next, the overall charge within the formula is balanced by employing a crisscross method. The cationic subscript is numerically matched to the anionic charge and vice versa, generating a charge-neutral chemical formula. 

All ionic compounds are named by following a systematic naming convention called the chemical nomenclature. The chemical formula helps determine the name of the compound. 

Binary ionic compounds name the metal cation first, succeeded by a parenthesized Roman-numeral indicating the metal charge if the metal can exist in different cationic states. The anion base-name ending with the suffix ‘–ide’ follows last. 

Consider the following three molecules. The cations are sodium or iron. The anions are iodide or bromide. Since sodium forms only a single type of cation, the parenthesized Roman-numeral is excluded. Iron forms two types of cations, which are displayed using parenthesized Roman-numerals. Thus, the names are sodium iodide, iron(II) bromide, and iron(III) bromide.

Polyatomic ionic compounds follow the same nomenclature rules, but apply the polyatomic anion name instead of the suffix ‘-ide’. Polyatomic anions containing oxygen, or oxyanions, are named based on the number of oxygen atoms. 

For example, multiple polyatomic ionic compounds are formed between sodium and oxyanions of bromine. 

The most common oxyanion of an element is named by combining the element’s base-name with the suffix ‘–ate’. The suffix changes to ‘–ite’ with one less oxygen, or it is given the prefix ‘per–’ with one oxygen atom more. 

Lastly, the oxyanion with one oxygen atom fewer than the oxyanion ending with ‘–ite’ obtains the prefix ‘hypo–’. Therefore the ionic compounds are sodium perbromate, sodium bromate, sodium bromite, and sodium hypobromite.

Hydrated ionic compounds or hydrates like CaCl₂∙2H₂O contain chemically-associated water molecules. The ionic part is named first. The number of water molecules is indicated using Greek-numeral-prefixes followed by the suffix ‘–hydrate.’ The chemical name of CaCl₂∙2H₂O is calcium chloride dihydrate.