4.5.2 Noble gas configurations under stress
It is remarkable how many molecules and ions of the typical elements can be represented by Lewis structures in which each atom has a noble gas shell structure. Nevertheless, many exceptions exist. According to the periodic trends summarised in Section 2, the highest fluorides of boron and phosphorus are BF3 and PF5. However, phosphorus, in accordance with Table 1, also forms the lower fluoride PF3. All three compounds are colourless gases at room temperature and contain the molecules BF3, PF3 and PF5. As the valency of fluorine is one (Table 1), each bond in these molecules is a shared electron pair, and we may write the Lewis structures as follows:
In how many of these Lewis structures do all the atoms have noble gas shell structures?
In only one, namely that of PF3; in Structure 5.11, the three shared electron pairs around the boron atom are two electrons short of the shell structure of neon, and in Structure 5.13, the five electron pairs around the phosphorus atom give us two electrons more than the shell structure of argon.
You will meet more exceptions of this sort in Section 6. Here, we merely note their existence, and observe that they are a consequence of our assumption that a chemical bond consists of two electrons shared between two atoms.