4.5 More about covalent bonding
So far, the valencies in Table 1 have just been numbers that we use to predict the formulae of compounds. But in the case of covalent substances they can tell us more. In particular, they can tell us how the atoms are linked together in the molecule. This information is obtained from a two-dimensional drawing of the structural formula of the molecule. (Note that structural formulae cannot be assumed to carry any implications about molecular shape.) Consider, for example, the molecules H2, Cl2, NCl3 and CH4. Their structural formulae are shown here as Structures 5.1-5.4. They can be drawn correctly by ensuring that the number of lines or bonds emerging from any atom is equal to its valency. Thus, in Structure 5.3, three bonds emerge from the nitrogen atom and one from each chlorine atom. The single lines of Structures 5.1-5.4 represent single bonds, but bonds may also be double or triple.
Check that you are comfortable with the bonding ideas discussed above by using the valencies of Table 1 to draw structural formulae for the following molecular substances: hydrogen chloride (HCl), ammonia (NH3), water (H2O), oxygen (O2), carbon dioxide (CO2), ethene (C2H4), hydrogen cyanide (HCN), ethyne (C2H2) and ethanal (CH3CHO).
Structures Q.1-Q.9 show the structural formulae. In each case, the number of lines issuing from each atom is equal to the element's quoted valency in Table 1.