Phosphorus is the correct answer.
It is the independent variable because you can ‘manipulate’ or ‘change’ it.
The Lewis structure of PF3 shows that the central phosphorus atom has one non bonding and three bonding electron pairs. In this compound each Phosphorus atom contributes 5 valence electrons while each fluoride contributes 7 valence electrons making a total of 26 valence electrons. The central Phosphorus atom forms single bonds with each of the fluoride atoms. Phosphorus, therefore ends up with a non-bonding pair since the fluoride atoms already have 8 electrons around them. <span />
