Chemical Compounds

Chemical compounds: names and masses

In Chemistry 102, you learnt about atomic masses. In this section we have learnt that atoms can combine to form compounds. Molecules are formed when atoms combine through covalent bonding, for example ammonia is a molecule made up of three hydrogen atoms and one nitrogen atom. The relative molecular mass (M) of ammonia \((\text{NH}_{3})\) is:

\begin{align*} M & = \text{ relative atomic mass of one nitrogen } + \text{ relative atomic mass of three hydrogens } \\ & = \text{14.0} + 3(\text{1.01}) \\ & = \text{17.03} \end{align*}

One molecule of \(\text{NH}_{3}\) will have a mass of \(\text{17.03}\) \(\text{units}\). When sodium reacts with chlorine to form sodium chloride, we do not get a molecule of sodium chloride, but rather a sodium chloride crystal lattice. Remember that in ionic bonding molecules are not formed. We can also calculate the mass of one unit of such a crystal. We call this a formula unit and the mass is called the formula mass. The formula mass for sodium chloride is:

\begin{align*} M & = \text{ relative atomic mass of one sodium atom } + \text{ relative atomic mass of one chlorine atom } \\ & = \text{23.0} + \text{35.45} \\ & = \text{58.45} \end{align*}

The formula mass for \(\text{NaCl}\) is \(\text{58.45}\) \(\text{units}\).