PF3 (phosphorus trifluoride) has one phosphorus atom and three fluorine atoms. In the lewis structure of PF3, there are three single bonds around the phosphorus atom, with three fluorine atoms attached to it. Each fluorine atom has three lone pairs, and the phosphorus atom has one lone pair.
Here’s how you can draw the PF3 lewis structure step by step.
Step #1: draw sketch
Step #2: mark lone pairs
Step #3: mark charges (if there are)
Let’s break down each step in detail.
#1 Draw Sketch
- First, determine the total number of valence electrons
Hence, phosphorus has five valence electrons and fluorine has seven valence electrons.
Since PF3 has one phosphorus atom and three fluorine atoms, so…
Valence electrons of one phosphorus atom = 5 × 1 = 5
Valence electrons of three fluorine atoms = 7 × 3 = 21
And the total valence electrons = 5 + 21 = 26
- Second, find the total electron pairs
We have a total of 26 valence electrons. And when we divide this value by two, we get the value of total electron pairs.
Total electron pairs = total valence electrons ÷ 2
So the total electron pairs = 26 ÷ 2 = 13
- Third, determine the central atom
We have to place the least electronegative atom at the center.
Since phosphorus is less electronegative than fluorine, assume that the central atom is phosphorus.
Therefore, place phosphorus in the center and fluorines on either side.
- And finally, draw the rough sketch
#2 Mark Lone Pairs
Here, we have a total of 13 electron pairs. And three P — F bonds are already marked. So we have to only mark the remaining ten electron pairs as lone pairs on the sketch.
Also remember that phosphorus is a period 3 element, so it can keep more than 8 electrons in its last shell. And fluorine is a period 2 element, so it can not keep more than 8 electrons in its last shell.
Always start to mark the lone pairs from outside atoms. Here, the outside atoms are fluorines.
So for each fluorine, there are three lone pairs, and for phosphorus, there is one lone pair.
Mark the lone pairs on the sketch as follows:
#3 Mark Charges
Use the following formula to calculate the formal charges on atoms:
Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons
For phosphorus atom, formal charge = 5 – 2 – ½ (6) = 0
For each fluorine atom, formal charge = 7 – 6 – ½ (2) = 0
Here, both phosphorus and fluorine atoms do not have charges, so no need to mark the charges.
And in the above structure, you can see that the central atom (phosphorus) forms an octet. Hence, the octet rule is satisfied.
Therefore, this structure is the stable lewis structure of PF3.
Next: PCl5 Lewis Structure