H3O+ Lewis Structure
Hydronium ion (oxonium) · What a proton actually is in water. Every pH measurement is a count of these.
The H3O+ Lewis structure has 8 valence electrons - the 1+ charge removes one, drawn as three bonds to hydrogen, with one lone pair in total, all inside square brackets. The central O is sp3 hybridized, which makes it a trigonal pyramidal ion with a bond angle of 107 degrees. The 1+ charge is written on the O, because that is the atom whose formal charge comes out positive - it is sharing more electrons than a free O atom owns. The charge belongs to the ion as a whole, which is what the brackets around the drawing mean.
- 8 valence e−
- trigonal pyramidal
- ≈107° bond angle
- sp3
- 1+ charge
H3O+ molecular geometry
Trigonal pyramidal, from 4 electron domains on the central O: 3 bonded atoms and 1 lone pair (AX3E1). The electron geometry is tetrahedral; the shape is what is left when the lone pairs are not counted as atoms. Wedges point toward you, hashed wedges away. Every shape in the VSEPR chart →
Calculated properties
| Total valence electrons | 8 |
|---|---|
| Bonding electrons | 6 (3 shared pairs) |
| Nonbonding electrons | 2 (1 lone pairs) |
| Lone pairs on O | 1 |
| Electron domains (steric number) | 4 |
| VSEPR notation | AX3E1 |
| Electron geometry | tetrahedral |
| Molecular geometry | trigonal pyramidal |
| Bond angle | ≈107° |
| Hybridization | sp3 |
| Charge | 1+ |
| Charge distribution | localised |
| Formal charges | O: +1 |
| Resonance structures | none |
| Molar mass | 19.023 g/mol |
H Hydrogen 3 atoms, 1 valence e− each O Oxygen 6 valence e−
How to draw the H3O+ 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.
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Skeleton. Join every connected pair with one bond. 3 bonds spends 6 electrons.
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Centre. Everything still unplaced goes on O, which now has 8 electrons around it.
The reasoning at each step
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Count the valence electrons
Add up the outer-shell electrons every atom brings, then account for the charge. A 1+ ion has lost one electron, so it is subtracted from it. Skip this and every count that follows will be wrong.
3 x 1 (H) + 6 (O) - 1 (1+ charge) = 8 valence electrons -
Choose the central atom
O is the least electronegative atom here, so it takes the centre. Hydrogen is never central: it can only form one bond.
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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.
3 bonds x 2 = 6 electrons used, 2 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 2, has 2, 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.
O: 1 (2 electrons) -
Check the central atom
O ends up with a full octet. Every electron in the budget is now placed.
O: 6 bonding + 2 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, and they are not the ion's charge either - but they must add up to it. Adding these up gives 1+, which is the check that the structure is complete.
O: 6 - 2 - 3 = +1 -
Work out the shape
Count the electron domains on O: 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 -
Decide whether it is polar
The O-H bonds are polar, and the lone pair on the central atom bends the molecule out of shape, so the dipoles do not cancel.
dipoles do not cancel -> polar
Where the 1+ charge sits
The 1+ charge is written on the O, because that is the atom whose formal charge comes out positive - it is sharing more electrons than a free O atom owns. The charge belongs to the ion as a whole, which is what the brackets around the drawing mean.
People also ask whether H3O+ is polar. Strictly it is neither: polar and nonpolar sort neutral molecules by whether their bond dipoles cancel. A 1+ ion is pulled towards any nearby charge whatever its shape, which is why its salts dissolve in water regardless of geometry. The table below is still worth reading - it shows how unevenly each bond holds its electrons, which is what decides whether the charge ends up spread or concentrated.
| Bond | Electronegativity difference | Character |
|---|---|---|
| O–H | 1.24 | polar covalent |
Common questions
How many valence electrons does H3O+ have?
H3O+ has 8 valence electrons. The atoms bring 9 between them, and the 1+ charge takes away one. 6 of them are in bonds and 2 sit in lone pairs.
Why does H3O+ have a 1+ charge?
The 1+ charge is written on the O, because that is the atom whose formal charge comes out positive - it is sharing more electrons than a free O atom owns. The charge belongs to the ion as a whole, which is what the brackets around the drawing mean. The brackets around the drawing are part of the answer: they say the charge belongs to the whole ion rather than to whichever atom happens to be nearest.
What is the molecular geometry of H3O+?
H3O+ is a trigonal pyramidal ion. The central O has 3 bonded groups and one lone pair, a steric number of 4, which gives tetrahedral electron geometry and a trigonal pyramidal ion.
What is the bond angle in H3O+?
The bond angle in H3O+ is 107 degrees. One lone pair pushes the three bonds down from the ideal 109.5 degrees.
What is the hybridization of H3O+?
The central O in H3O+ is sp3 hybridized. Steric number 4 means 4 orbitals have to be mixed, which is exactly what sp3 gives you.
Is H3O+ polar or nonpolar?
Neither, strictly. Polar and nonpolar describe neutral molecules, by asking whether their bond dipoles cancel out. H3O+ carries a net 1+ charge, so it is pulled towards any nearby charge whatever its shape - which is why ionic compounds dissolve in water regardless of geometry. The question the shape does answer is whether the charge is spread evenly: it is not, because the O-H bonds are polar, and the lone pair on the central atom bends the molecule out of shape, so the dipoles do not cancel.