Question

Chlorine and oxygen form many different compounds, including ClO2 and Cl2O3. How do the names of these compounds differentiate one chlorine oxide from another?

Answer 1

Binary molecular compounds of chlorine and oxygen are usually told apart by either
- using Greek prefixes to indicate how many atoms of each element are present, or
- giving the oxidation state of chlorine in parentheses.

Rules (common/IUPAC molecular naming)
- Name the less electronegative element (chlorine) first, then the more electronegative element (oxygen) with the -ide ending.
- Use prefixes to show the number of atoms: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, etc. (the prefix mono- is often omitted on the first element).
- Examples:
- ClO2 = chlorine dioxide (prefix form). It can also be called chlorine(IV) oxide (Cl is +4).
- Cl2O3 = dichlorine trioxide (prefix form). Calling it dichlorine(III) oxide would indicate an average oxidation state of +3 per Cl; in some structures the chlorines may actually have different oxidation states, so the prefix name is less ambiguous.
- Cl2O = dichlorine monoxide (also named chlorine(I) oxide, each Cl +1).
- Cl2O7 = dichlorine heptoxide (chlorine(VII) oxide, Cl +7).

So, to distinguish one chlorine oxide from another, chemists most often use the numeric prefixes (dichlorine trioxide, chlorine dioxide, etc.), or specify the chlorine oxidation state in parentheses when that is important (chlorine(IV) oxide).