Taking into account the definition of valence electrons and Lewis structure, the lewis structure of PF₃ shows that the central phosphorus atom has 1 nonbonding and 3 bonding electron pair(s).
The valence electrons are the electrons found in the highest energy levels of the atom, being these responsible for the interaction between atoms of different species or between the atoms of the same. That is, the electrons in the external energy levels are those that will be used in the formation of compounds and which are called valence electrons.
Lewis structure, also called a dot diagram, Lewis model or Lewis representation, is a graphical representation that shows the bonds between the atoms of a molecule and the lone electron pairs that may exist.
That is, the Lewis structures represent the bonds between the atoms of a molecule, as well as any unbonded pair of electrons and is used to know the number of valence electrons of an element that interact with others or between its same species.
In the Lewis representation around the element symbol of the periodic table points or asterisks to indicate the external electrons they have.
In this case you have the molecule phosphorous trifluoride, PF₃.
Phosphorous has '5' valence electrons and fluorine has '7' valence electrons. Therefore, the total number of valence electrons in phosphorous trifluoride, PF₃= 5 + 3×7 = 26
Generally the least electronegative atoms are those that make up central atoms.
So each fluorine atom is attached to the phosphorous atom, forming three P-F bonds.
The central phosphorous atom needs 3 electrons to complete the octet, while each phosphor needs 1 electron to complete it.
So the Lewis structure is determined as shown in the attached image.
You can see that according to electron-dot structure, there are 6 number of bonding electrons and 20 number of non-bonding electrons.
Finally, the lewis structure of PF₃ shows that the central phosphorus atom has 1 nonbonding and 3 bonding electron pair(s).
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