How To Name Covalent Compounds

Naming compounds in chemistry is one of the fun parts. Naming conventions follow a set of rules with descriptive words. Once you know the rules, you can name any compound that you come across. And likewise, you can easily tell by the name of any compound what its structure is.

With new compounds discovered all the time, and millions already in existence, there would be no way to sort everything out without a consistent naming structure. Let's look at some of the concepts around how to name covalent compounds.

First of all, to name a covalent compound, it helps to know what a covalent compound is. Covalent compounds are formed when two or more nonmetal atoms bond by sharing valence electrons. Valence electrons are the outermost electrons of an atom. Elements want to fill up their electron orbitals, or shells, with electrons, so they will bond with other atoms that allow them to do this. In the word 'covalent,' 'co' means share and 'valent' means valence electrons.

It's worth noting that organic chemistry has a completely different nomenclature.

Greek prefixes are used for naming compounds based on the elemental subscript which provides the number of atoms in the compound.

Number Prefix

1 mono-
2 di-
3 tri-
4 tetra-
5 penta-
6 hexa-
7 hepta-
8 octa-
9 nona-
10 deca-

For example, SF₄ is sulfur tetrafluoride. The tetra- prefix on the fluoride stem name indicates that there are 4 fluoride atoms in this compound. Generally, the first atom when read left to right is the least abundant in the compound.

First, give the name of the first atom in the compound. Then give the Greek prefix for the number of the second atom. Then name the second atom and end it with -ide.

Ionic compounds are composed of ions. Most ionic compounds contain metal and nonmetal atoms. If the compounds are positively charged they are known as cations. If the compounds are negatively charged they are known as anions.

An ionic compound is named by first giving the name of the cation followed by the name of the anion. For example, sodium (Na+) and chloride (Cl−) together form sodium chloride.

A polyatomic ion is an ion consisting of two or more atoms. One charged atom is called a monoatomic ion, while a binary covalent compound is composed of two non-metal atoms. Because the molecule is an ion, it means it has an overall electrical charge.

Many ions are oxoanions, which means oxygen is combined with another element. The naming convention follows a specific formula. The 'stem' is the element name.

The -ate form: for example, sulfate, SO₄^2-

The -ite form has one less oxygen that the -ate form: sulfite, SO₃^2-

These suffixes -ate and -ite denote the relative number of oxygen atoms in the oxoanion. Other suffixes and prefixes differentiate other possibilities:

The hypo- stem -ite has two less oxygens than the -ate form: hypochlorite, ClO-

The per- stem -ate form has one more oxygen than the -ate form: perchlorate, ClO₄-

The -ide form is the monatomic anion: chloride, Cl−

The suffixes do not tell you the actual number of oxygens, however, just the relative number.

The prefix thio- in a compound means that an oxygen atom has been replaced by a sulfur atom.

Naming covalent compounds with three elements follows these similar rules. As you would in the other cases, specify the formula, charge and number of each ion.

For example, lithium hydrogen phosphate contains three elements: lithium, which is a cation, and hydrogen phosphate. Therefore its name is Li₄HPO₄.

Likewise, Na₂SO₄ refers to sodium sulfate.

The naming conventions of polyatomic ions must either be remembered or be referred to when writing the formula of the compound. The first step is to identify the elemental cation and anion, and then name them. The cation is named first, then the second part of the name is the anion and its known or deduced charge.

For instance, Mg₃N₂ refers to magnesium nitride, because magnesium is the cation, and nitrogen can be deduced as forming an anion of charge equal to the group number minus 8, which is N^3-, the nitride ion.