H3PO4 Lewis Structure
Phosphoric acid · The tang in cola, and a fertiliser feedstock.
The H3PO4 Lewis structure has 32 valence electrons, drawn as four P–O bonds and three bonds to hydrogen, with nine lone pairs in total. The central P is sp3 hybridized, which makes it a tetrahedral molecule with a bond angle of 109.5 degrees. H3PO4 is polar: the P-O bond dipoles do not cancel in this shape, leaving a net dipole.
- 32 valence e−
- tetrahedral
- 109.5° bond angle
- sp3
- polar
Calculated properties
| Total valence electrons | 32 |
|---|---|
| Bonding electrons | 14 (7 shared pairs) |
| Nonbonding electrons | 18 (9 lone pairs) |
| Lone pairs on P | 0 |
| Electron domains (steric number) | 4 |
| VSEPR notation | AX4 |
| Electron geometry | tetrahedral |
| Molecular geometry | tetrahedral |
| Bond angle | 109.5° |
| Hybridization | sp3 |
| Polarity | polar |
| Formal charges | P: +1, O: -1 |
| Resonance structures | none |
| Molar mass | 97.994 g/mol |
How to draw the H3PO4 Lewis structure
Step through it. The bar shows how much of the 32-electron budget is spent at each stage - it never grows, which is the whole constraint.
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Skeleton. Join every connected pair with one bond. 7 bonds spends 14 electrons.
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Outer octets. Spend electrons as lone pairs on the outer atoms. That uses the entire budget, and the central atom has nothing left over.
The reasoning at each step
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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.
3 x 1 (H) + 5 (P) + 4 x 6 (O) = 32 valence electrons -
Work out what is bonded to what
This is an oxyacid. Every oxygen bonds to the central P, and the acidic hydrogens sit on oxygen, not on P.
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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.
7 bonds x 2 = 14 electrons used, 18 left -
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 18, has 18, exactly enough -
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.
O1: 2, O2: 2, O3: 2, O4: 3 (18 electrons) -
Check the central atom
P ends up with a full octet. Every electron in the budget is now placed.
P: 8 bonding + 0 nonbonding = 8 -
Check the formal charges
Formal charge is valence electrons, minus lone-pair electrons, minus the number of bonds. They are not real charges, but they have to add up to the overall charge on the species, and a structure that keeps them small is the better one.
P: 5 - 0 - 4 = +1 ; O4: 6 - 6 - 1 = -1 -
Work out the shape
Count the electron domains on P: 4 bonded groups. 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 tetrahedral arrangement.
steric number 4 -> sp3 -> tetrahedral, bond angle 109.5 -
Decide whether it is polar
The P-O bond dipoles do not cancel in this shape, leaving a net dipole.
dipoles do not cancel -> polar
The expanded-octet version
Courses disagree about H3PO4. The structure at the top of this page obeys the octet rule and leaves formal charges behind. The one below puts more than eight electrons on the central atom and brings every formal charge to zero. Check which convention your course uses.
Is H3PO4 polar or nonpolar?
H3PO4 is polar. The P-O bond dipoles do not cancel in this shape, leaving a net dipole.
| Bond | Electronegativity difference | Character |
|---|---|---|
| P–O | 1.25 | polar covalent |
| O–H | 1.24 | polar covalent |
Common questions
How many valence electrons does H3PO4 have?
H3PO4 has 32 valence electrons. 14 of them are in bonds and 18 sit in lone pairs.
What is the molecular geometry of H3PO4?
H3PO4 is a tetrahedral molecule. The central P has 4 bonded groups, a steric number of 4, which gives tetrahedral electron geometry and a tetrahedral molecule.
What is the bond angle in H3PO4?
The bond angle in H3PO4 is 109.5 degrees. That is the ideal tetrahedral angle, and nothing distorts it here.
What is the hybridization of H3PO4?
The central P in H3PO4 is sp3 hybridized. Steric number 4 means 4 orbitals have to be mixed, which is exactly what sp3 gives you.
Is H3PO4 polar or nonpolar?
H3PO4 is polar. The P-O bond dipoles do not cancel in this shape, leaving a net dipole.
Does H3PO4 obey the octet rule?
It can be drawn either way, and different courses teach different conventions. The octet structure keeps eight electrons on every atom but leaves formal charges behind. The expanded-octet structure puts more than eight electrons on the central atom and brings every formal charge to zero. Both are shown on this page.