1.6 Noble gas configurations under stress
The elements phosphorus and boron form fluorides having formulas BF3, PF3 and PF5.
Write the Lewis structures for BF3, PF3 and PF5.
As the valency of fluorine is one, each bond in BF3, PF3 and PF5 is a shared electron pair, so the Lewis structures may be written:
In how many of these Lewis structures do all the atoms have noble gas shell structures?
In only one, namely PF3.
Looking first at BF3, you can see around the boron atom there are six electrons – it is two electrons short of the shell structure of neon. In contrast there are five electron pairs around the phosphorus atom in PF5, which is two more than the shell structure of argon.
Clearly things aren’t as neat and tidy as they first appeared.