PH3 (phosphine) has one phosphorus atom and three hydrogen atoms. In the lewis structure of PH3, there are three single bonds around the phosphorus atom, with three hydrogen atoms attached to it, and on the phosphorus atom, there is one lone pair.
Here’s how you can draw the PH3 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 hydrogen has one valence electron.
Since PH3 has one phosphorus atom and three hydrogen atoms, so…
Valence electrons of one phosphorus atom = 5 × 1 = 5
Valence electrons of three hydrogen atoms = 1 × 3 = 3
And the total valence electrons = 5 + 3 = 8
- Second, find the total electron pairs
We have a total of 8 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 = 8 ÷ 2 = 4
- Third, determine the central atom
Here hydrogen can not be the central atom. Because the central atom is bonded with at least two other atoms, and hydrogen has only one electron in its last shell, so it can not make more than one bond.
Hence, here we have to assume that the central atom is phosphorus.
Therefore, place phosphorus in the center and hydrogens on either side.
- And finally, draw the rough sketch
#2 Mark Lone Pairs
Here, we have a total of 4 electron pairs. And three P — H bonds are already marked. So we have to only mark the remaining one electron pair as a lone pair 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 hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell.
Always start to mark the lone pairs from outside atoms. Here, the outside atoms are hydrogens. But no need to mark on hydrogen, because each hydrogen has already two electrons.
So for phosphorus, there is one lone pair.
Mark the lone pair 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 hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0
Here, both phosphorus and hydrogen 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 PH3.
Next: NO Lewis Structure