Naming Molecular Compounds


Learning Objective

  • Apply the rules for naming molecular compounds

Key Points

    • In nomenclature of simple molecular compounds, the more electropositive atom is written first and the more electronegative element is written last with an -ide suffix.
    • The Greek prefixes are used to dictate the number of a given element present in a molecular compound.
    • Prefixes can be shortened when the ending vowel of the prefix “conflicts” with a starting vowel in the compound.
    • Common exceptions exist for naming molecular compounds, where trivial or common names are used instead of systematic names, such as ammonia (NH3) instead of nitrogen trihydride or water (H2O) instead of dihydrogen monooxide.


  • nomenclatureA set of rules used for forming the names or terms in a particular field of arts or sciences.
  • electronegativeTending to attract electrons within a chemical bond.
  • electropositiveTending to not attract electrons (repel) within a chemical bond.

Chemical Nomenclature

The primary function of chemical nomenclature is to ensure that a spoken or written chemical name leaves no ambiguity concerning to what chemical compound the name refers. Each chemical name should refer to a single substance. Today, scientists often refer to chemicals by their common names: for example, water is not often called dihydrogen oxide. However, it is important to be able to recognize and name all chemicals in a standardized way. The most widely accepted format for nomenclature has been established by IUPAC.

Molecular compounds are made when two or more elements share electrons in a covalent bond to connect the elements. Typically, non-metals tend to share electrons, make covalent bonds, and thus, form molecular compounds.

Rules for Naming Molecular Compounds:

  1. Remove the ending of the second element, and add “ide” just like in ionic compounds.
  2. When naming molecular compounds prefixes are used to dictate the number of a given element present in the compound. ” mono-” indicates one, “di-” indicates two, “tri-” is three, “tetra-” is four, “penta-” is five, and “hexa-” is six, “hepta-” is seven, “octo-” is eight, “nona-” is nine, and “deca” is ten.
  3. If there is only one of the first element, you can drop the prefix. For example, CO is carbon monoxide, not monocarbon monoxide.
  4. If there are two vowels in a row that sound the same once the prefix is added (they “conflict”), the extra vowel on the end of the prefix is removed. For example, one oxygen would be monooxide, but instead it’s monoxide. The extra o is dropped.

Generally, the more electropositive atom is written first, followed by the more electronegative atom with an appropriate suffix. For example, H2O (water) can be called dihydrogen monoxide (though it’s not usually). Organic molecules (molecules made of C and H along with other elements) do not follow this rule.

Examples of Molecular Compound Names:

  • SO2 is called sulfur dioxide
  • SiI4 is called silicon tetraiodide
  • SF6 is called sulfur hexafluoride
  • CS2 is called carbon disulfide
Naming Compounds – Part 2 – YouTubeThis video explains how to use a chemical name to write the formula for that compound.