PH₃ Lewis Structure

Phosphine · A toxic gas used as a fumigant and in semiconductor manufacturing.

• 8 valence e−
• trigonal pyramidal
• 107° bond angle
• sp³
• polar

Calculated properties

How to draw the PH₃ Lewis structure

Step through it. The bar shows how much of the 8-electron budget is spent at each stage - it never grows, which is the whole constraint.

The reasoning at each step

1. Count the valence electrons

   Add up the outer-shell electrons every atom brings. That total is the budget for the whole structure - every line and every dot has to come out of it, and nothing may be added.

   5 (P) + 3 x 1 (H) = 8 valence electrons

2. Choose the central atom

   P is the least electronegative atom here, so it takes the centre. Hydrogen is never central: it can only form one bond.

3. Join everything with single bonds first

   Every connection starts as one shared pair. Draw them all before worrying about double bonds - the arithmetic in the next step is what tells you where the double bonds have to go.

   3 bonds x 2 = 6 electrons used, 2 left

4. Work out how far short you are

   Add up what every atom still needs to fill its shell - eight electrons for most atoms, two for hydrogen - minus what its single bonds already give it. Compare that with the electrons you have left. The difference decides everything that follows.

   needs 2, has 2, exactly enough

5. Fill in the lone pairs

   Every electron not in a bond sits as a lone pair on an atom. Each atom takes exactly what it needs to finish its shell - there is no choice left at this point.

   P: 1 (2 electrons)

6. Check the central atom

   P ends up with a full octet. Every electron in the budget is now placed.

   P: 6 bonding + 2 nonbonding = 8

7. Check the formal charges

   Formal charge is valence electrons, minus lone-pair electrons, minus the number of bonds. Every atom here comes out at zero, which is the sign of a good structure.

   all formal charges = 0

8. Work out the shape

   Count the electron domains on P: 3 bonded groups plus 1 lone pair. A double or triple bond still counts as one domain, because it points in one direction. That gives tetrahedral electron geometry; ignore the lone pairs and the atoms themselves sit in a trigonal pyramidal arrangement. One lone pair pushes the three bonds down from the ideal 109.5 degrees.

   steric number 4 -> sp3 -> trigonal pyramidal, bond angle 107

9. Decide whether it is polar

   The individual bonds are close to nonpolar, but the lone pair sits on one side of the molecule, so there is still a net dipole.

   dipoles do not cancel -> polar

Is PH₃ polar or nonpolar?

PH₃ is polar. The individual bonds are close to nonpolar, but the lone pair sits on one side of the molecule, so there is still a net dipole.

Common questions

How many valence electrons does PH₃ have?

PH₃ has 8 valence electrons. 6 of them are in bonds and 2 sit in lone pairs.

What is the molecular geometry of PH₃?

PH₃ is a trigonal pyramidal molecule. The central P has 3 bonded groups and one lone pair, a steric number of 4, which gives tetrahedral electron geometry and a trigonal pyramidal molecule.

What is the bond angle in PH₃?

The bond angle in PH₃ is 107 degrees. One lone pair pushes the three bonds down from the ideal 109.5 degrees.

What is the hybridization of PH₃?

The central P in PH₃ is sp³ hybridized. Steric number 4 means 4 orbitals have to be mixed, which is exactly what sp³ gives you.

Is PH₃ polar or nonpolar?

PH₃ is polar. The individual bonds are close to nonpolar, but the lone pair sits on one side of the molecule, so there is still a net dipole.

Related molecules

• NH₃ also trigonal pyramidal
• N₂H₄ also trigonal pyramidal
• H₃PO₄ also built around P
• PCl₅ also built around P
• CH₄ also sp3
• CCl₄ also sp3