Naming Covalent Molecular Compounds

Covalent (molecular) compounds are formed from two or more nonmetals that share electrons, and they are named with Greek prefixes that give the number of each atom, such as carbon dioxide (CO₂), dinitrogen tetroxide (N₂O₄) and sulfur hexafluoride (SF₆). In brief: name the first element, add a prefix to the second element and change its ending to “-ide”; do not use “mono-” on the first element, but use it on the second, as in carbon monoxide.

Unlike ionic compounds, the same two nonmetals can combine in several ratios, so the name must show how many atoms of each element are in the molecule. This article gives the prefixes and the rules for naming, explains how to decide between ionic and covalent naming, lists common compounds that keep their traditional names, works through examples in both directions and describes the history of the prefix system.

How do you know that a compound is molecular?

A compound made of two nonmetals is usually molecular. A compound of a metal and a nonmetal is usually ionic. Look at the elements in the formula: CO₂ has carbon and oxygen, both nonmetals, so it is molecular. NaCl has a metal and a nonmetal, so it is ionic. Compounds with a polyatomic ion such as ammonium are ionic, even if all the elements are nonmetals. Hydrogen compounds with nonmetals, such as HCl and H₂O, are molecular, and some have special names.

What are the numerical prefixes?

What are the naming rules?

1. Write the name of the element that is further left (or lower) in the periodic table first. In the formula, this element is usually written first as well. Exception: oxygen is written after all the other elements except fluorine.
2. Add a prefix to the first element only if there is more than one atom of it. Do not use “mono-” on the first element: CO₂ is carbon dioxide, not monocarbon dioxide.
3. Name the second element with the stem and the ending “-ide”, and always add the prefix, including “mono-” if there is one atom.
4. Before “oxide”, the final “o” or “a” of the prefix is dropped to avoid a clumsy double vowel: mono + oxide gives monoxide (not monooxide), tetra + oxide gives tetroxide, penta + oxide gives pentoxide and hepta + oxide gives heptoxide. Prefixes ending in “i” (di-, tri-) are written in full: dioxide and trioxide.

Examples of molecular compounds

Which compounds have common names?

Some molecular compounds are always called by their traditional names, and prefixes are not used.

Worked examples

1. Name N₂O₄. Two nitrogen atoms (di-nitrogen), four oxygen atoms (tetr-oxide). The name is dinitrogen tetroxide.
2. Name SiO₂. One silicon atom (no prefix), two oxygen atoms (dioxide). The name is silicon dioxide, although in solid form it is a network, not molecules.
3. Formula of diphosphorus pentoxide. Di- means 2 phosphorus atoms. Penta- means 5 oxygen atoms. The formula is P₂O₅ (the actual molecule is P₄O₁₀, and the formula P₂O₅ is the empirical formula).
4. Formula of xenon tetrafluoride. Xenon has no prefix, which means 1; tetra-fluoride means 4 fluorine atoms. The formula is XeF₄.
5. Ionic or molecular? Name Fe₂O₃ and N₂O₃. Fe₂O₃ has a metal, so it is ionic: iron(III) oxide. N₂O₃ has two nonmetals, so it is molecular: dinitrogen trioxide.
6. Molar mass. Sulfur hexafluoride, SF₆: 32.07 + 6 × 19.00 = 146.07 g/mol. The mass percent of fluorine is 114.00 ÷ 146.07 × 100% = 78.0%.

Why are nitrogen and its oxides a good example?

Nitrogen forms several oxides with different properties, and the prefixes are necessary to tell them apart: N₂O (dinitrogen monoxide) is laughing gas, NO (nitrogen monoxide) is a colorless gas that reacts with oxygen to form NO₂ (nitrogen dioxide), a brown toxic gas, N₂O₃ (dinitrogen trioxide) is a blue liquid at low temperature, N₂O₄ (dinitrogen tetroxide) is in equilibrium with NO₂, and N₂O₅ (dinitrogen pentoxide) is a white solid. Their oxidation numbers for nitrogen are +1, +2, +4, +3, +4 and +5.

How did the system develop?

The Greek prefix method was developed in the 19th and early 20th centuries as chemists realized that the same elements could combine in more than one proportion. Dalton’s law of multiple proportions, published in 1803, explained this: if two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers. The prefix system was adopted into the international rules of IUPAC in the 20th century and is part of the standard inorganic nomenclature.

Which mistakes are common?

Mistake 1: Adding “mono-” to the first element

Say carbon dioxide, not monocarbon dioxide. But do say carbon monoxide.

Mistake 2: Using prefixes for ionic compounds

Prefixes are for molecular compounds. Sodium chloride is not “sodium monochloride”.

Mistake 3: Using prefixes for compounds with common names

Say water and ammonia, not dihydrogen monoxide and nitrogen trihydride, in normal usage.

Mistake 4: Giving the wrong order of elements

The less electronegative element comes first: carbon dioxide, not dioxygen carbide.