Nomenclature of Molecular Compounds

How to Name Molecular Compounds

– Unlike ionic compounds, Molecular compounds contain discrete molecular units.

– They are usually composed of nonmetallic elements.

– Many molecular compounds are binary compounds.

– Naming binary molecular compounds is similar to naming binary ionic compounds.

– We place the name of the first element in the formula first, and the second element is named by adding “-ide” to the root of the element name.

– Some examples are:

HCl   Hydrogen chloride

SiC    Silicon carbide

HBr  Hydrogen bromide

– It is quite common for one pair of elements to form several different compounds.

– In these cases, confusion in naming the compounds is avoided by the use of Greek prefixes to denote the number of atoms of each element present (see Table).

– Consider these examples:

CO       Carbon monoxide

SO₃      Sulfur trioxide

CO₂      Carbon dioxide

NO₂     Nitrogen dioxide

SO₂     Sulfur dioxide

N₂O₄  Dinitrogen tetroxide

A guideline for naming Molecular compounds with prefixes

– These guidelines are helpful when you are naming compounds with prefixes:

– The prefix “mono-” may be omitted for the first element.

– For example, PCl₃ is named phosphorus trichloride, not monophosphorus trichloride.

– Thus, the absence of a prefix for the first element usually means that only one atom of that element is present in the molecule.

– For oxides, the ending “a” in the prefix is sometimes omitted.

– For example, N₂O₄ may be called dinitrogen tetroxide rather than dinitrogen tetraoxide.

– Exceptions to the use of Greek prefixes are molecular compounds containing hydrogen.

– Traditionally, many of these compounds are called either by their common, non-systematic names or by names that do not specifically indicate the number of H atoms present:

B₂H₆         Diborane

PH₃          Phosphine

CH₄          Methane

H₂O           Water

SiH₄          Silane

H₂S         Hydrogen sulfide

NH₃         Ammonia

– Note that even the order of writing the elements in the formulas is irregular.

– These examples show that H is written first in water and hydrogen sulfide, whereas H is written last in the other compounds.

– Writing formulas for molecular compounds is usually straightforward.

– Thus, the name arsenic trifluoride means that there is one As atom and three F atoms in each molecule and the molecular formula is AsF₃.

– Note that the order of elements in the formula is the same as that in its name.

– The figure below summarizes the steps for naming ionic and molecular compounds:

Solved Problems

Example (1): Name the following these compounds:

(a) PBr₅

(b) As₂O₅

Solution

(a) Because there are five bromine atoms present, the compound is phosphorus pentabromide.

(b) There are two arsenic atoms and five oxygen atoms present, so the compound is diarsenic pentoxide.

– Note that the “a” is omitted in “Penta.

Example (1): Write chemical formulas for the following molecular compounds:

(a) bromine trifluoride

(b) diboron trioxide

Solution

(a) Because there are three fluorine atoms and one bromine atom present, the formula is BrF₃.

(b) There are two boron atoms and three oxygen atoms present, so the formula is B₂O₃.

Reference: General Chemistry: The Essential Concepts / Raymond Chang, Jason Overby. (sixth edition) .