Covalent compounds form when nonmetal atoms share electrons, and their names tell you which elements are present and how many atoms of each element are in one molecule. Because many pairs of nonmetals can combine in more than one ratio, the name must include number clues. Greek prefixes such as mono-, di-, and tri- make the formula and name match clearly.
Learning this system helps you read chemical formulas, write correct names, and communicate precisely in chemistry.
Understanding Chemistry: Naming Covalent Compounds
A covalent name is a compact record of a molecule's composition. The prefixes matter because changing the number of atoms can create a completely different substance. Carbon monoxide has one oxygen atom per carbon atom.
Carbon dioxide has two oxygen atoms per carbon atom. These substances behave differently in the body and in the environment. Nitrogen monoxide, nitrogen dioxide, and dinitrogen tetroxide are further examples of substances made from the same two elements in different ratios.
A name therefore does more than label a sample. It gives information needed to build the correct formula and predict what material is being discussed.
When writing a formula from a name, treat each prefix as an instruction for the number of atoms that follows it. Sulfur hexafluoride contains one sulfur atom and six fluorine atoms. The missing prefix on sulfur means one, because the first element follows a special naming convention.
For phosphorus pentachloride, phosphorus has one atom and chlorine has five atoms. The ending ide identifies the second element in a two element molecular compound.
It does not mean that the substance has ions. This point is important because chloride in a covalent name does not make the compound ionic.
Students often confuse covalent naming with ionic naming because both use element names and the ide ending. Ionic compounds use positive and negative ions, so their formulas are based on charge balance. Their names do not use Greek prefixes to show atom counts.
For example, sodium chloride is named from sodium ions and chloride ions. A molecular compound made only from nonmetals uses prefixes instead. Before naming a substance, first check the kinds of elements present.
A metal paired with a nonmetal usually signals ionic naming. Two nonmetals usually signal molecular naming. This first decision prevents many later mistakes.
Pronunciation rules make some names sound smoother, but they do not change the atom count. One oxygen becomes monoxide rather than monooxide. Five oxygen atoms become pentoxide rather than pentaoxide.
Students should learn these as spelling adjustments, not as new prefixes. It helps to work in a fixed order. Identify the first element, count its atoms, identify the second element, count its atoms, then apply the ending change and pronunciation adjustment.
Read each completed name back into a formula to check it. This habit catches reversed elements, missing prefixes, and incorrect endings before they become repeated errors.
Key Facts
- Binary covalent compounds contain two different nonmetal elements.
- Name the first element first, using a prefix only if there is more than one atom of that element.
- Name the second element with a Greek prefix and change its ending to -ide.
- Common prefixes: 1 = mono, 2 = di, 3 = tri, 4 = tetra, 5 = penta, 6 = hexa, 7 = hepta, 8 = octa, 9 = nona, 10 = deca.
- Do not use mono- on the first element, so CO is carbon monoxide, not monocarbon monoxide.
- Drop a final a or o from a prefix before oxide when it makes pronunciation easier, such as monoxide and pentoxide.
Vocabulary
- Covalent compound
- A compound made of atoms, usually nonmetals, that are held together by shared electron pairs.
- Binary compound
- A compound that contains exactly two different elements.
- Greek prefix
- A word part such as di-, tri-, or tetra- that shows the number of atoms of an element in a molecular formula.
- Molecular formula
- A formula that shows the actual number of each type of atom in one molecule of a compound.
- -ide ending
- The ending used on the second element name in a binary compound, such as chloride, oxide, or sulfide.
Common Mistakes to Avoid
- Using mono- on the first element is wrong because the first element does not get mono- when only one atom is present. CO is carbon monoxide, not monocarbon monoxide.
- Forgetting the -ide ending on the second element is wrong because binary covalent names require the second element to be modified. N2O4 is dinitrogen tetroxide, not dinitrogen tetraoxygen.
- Using ionic naming rules for two nonmetals is wrong because covalent compounds use prefixes instead of charge balancing. PCl5 is phosphorus pentachloride, not phosphorus(V) chloride in the basic prefix system.
- Keeping awkward double vowels in oxide names is often wrong because prefix vowels are commonly dropped for smoother pronunciation. N2O5 is dinitrogen pentoxide, not dinitrogen pentaoxide.
Practice Questions
- 1 Write the name of N2O3 and identify the prefix used for each element.
- 2 Write the formula for sulfur hexafluoride and state how many fluorine atoms are in one molecule.
- 3 Explain why CO2 is named carbon dioxide but CO is named carbon monoxide, focusing on the purpose of prefixes.