Water

H₂O · water

Oxygen holds two bonding pairs and two lone pairs. The lone pairs repel more strongly than the bonds, compressing the H–O–H angle from 109.5° to 104.5° and making water bent and polar.

Valence electrons: 8
Charge: 0
Lone pairs: 2
Molecular geometry: bent

Lines represent covalent bonds (shared electron pairs); dots represent lone pairs (nonbonding electrons).
A single bond is one shared pair of electrons, a double bond two shared pairs, and a triple bond three shared pairs.
Each atom in a bond contributes to the shared pair and counts the shared electrons toward its own octet. The atoms do not each gain two extra electrons; they share the same pair, and both atoms count it.

Count

Start from the -electron total, because it sets how many electrons the whole structure may contain: each hydrogen is a Group 1 element, so it has 1 valence electron, and with 2 of them that is 1 × 2 = 2; oxygen is a Group 16 element, so it has 16 - 10 = 6 valence electrons, for 8 valence electrons in all; finishing with more or fewer than 8 would mean a counting error was made.

Build

Oxygen is the only that can form more than one bond, so it must be the . A hydrogen atom has a single and therefore forms exactly one bond, which means hydrogen is always a .
Begin the structure with a from O to each of the 2 terminal . Each single bond is a shared pair, so these bonds together use 4 electrons and 4 of the 8 are still available.
The terminal are all hydrogen, and hydrogen needs only two electrons to fill its 1s shell, which the in its single bond already provides. The 4 remaining electrons therefore all belong to O.
Assign the remaining 4 electrons to the as 2 on O. All 8 are now placed, so the count returns to zero.

Verify

Verify each in turn, counting shared and together. O: 4 shared + 4 nonbonding = 8 (a full ); each H: 2 shared + 0 nonbonding = 2 (a duet, all hydrogen needs).
Check the on each . Formal charge equals an atom's , minus its nonbonding (lone-pair) electrons, minus half of its ; the bonding electrons are halved because the two atoms in a bond share that pair equally, so each is assigned one electron of it. Here all 3 atoms have a formal charge of zero, which is the ideal outcome and confirms this is the preferred structure.

Watch out

Don't let a tidy drawing fool you into calling H2O symmetric: the on O bend it into a shape, and that bend is exactly what makes it .

Shape

Apply by counting the around O: 2 bonding domains plus 2 gives 4 electron domains. Domains positioned to minimize their mutual repulsion adopt , which corresponds to sp³ .
A holds its ground but never shows up in the we name. Trace only the and this is bent, angles pinched to about ≈104.5° by the extra .
: because the is bent, the bond pulls don't cancel; they add up. The electron-rich end sits toward O, giving the a permanent dipole. Polar.

Worth knowing

That bend is doing a lot of work. Because water is , its two bond dipoles add up instead of canceling, and that little is why water dissolves , why ice floats, and why a microwave can heat your soup.

Count

Start from the -electron total, because it sets how many electrons the whole structure may contain: each hydrogen is a Group 1 element, so it has 1 valence electron, and with 2 of them that is 1 × 2 = 2; oxygen is a Group 16 element, so it has 16 - 10 = 6 valence electrons, for 8 valence electrons in all; finishing with more or fewer than 8 would mean a counting error was made.

Build

Oxygen is the only that can form more than one bond, so it must be the . A hydrogen atom has a single and therefore forms exactly one bond, which means hydrogen is always a .

Begin the structure with a from O to each of the 2 terminal . Each single bond is a shared pair, so these bonds together use 4 electrons and 4 of the 8 are still available.

The terminal are all hydrogen, and hydrogen needs only two electrons to fill its 1s shell, which the in its single bond already provides. The 4 remaining electrons therefore all belong to O.

Assign the remaining 4 electrons to the as 2 on O. All 8 are now placed, so the count returns to zero.

Verify

Verify each in turn, counting shared and together. O: 4 shared + 4 nonbonding = 8 (a full ); each H: 2 shared + 0 nonbonding = 2 (a duet, all hydrogen needs).

Check the on each . Formal charge equals an atom's , minus its nonbonding (lone-pair) electrons, minus half of its ; the bonding electrons are halved because the two atoms in a bond share that pair equally, so each is assigned one electron of it. Here all 3 atoms have a formal charge of zero, which is the ideal outcome and confirms this is the preferred structure.

Watch out

Don't let a tidy drawing fool you into calling H2O symmetric: the on O bend it into a shape, and that bend is exactly what makes it .

Shape

Apply by counting the around O: 2 bonding domains plus 2 gives 4 electron domains. Domains positioned to minimize their mutual repulsion adopt , which corresponds to sp³ .

A holds its ground but never shows up in the we name. Trace only the and this is bent, angles pinched to about ≈104.5° by the extra .

: because the is bent, the bond pulls don't cancel; they add up. The electron-rich end sits toward O, giving the a permanent dipole. Polar.

Worth knowing

That bend is doing a lot of work. Because water is , its two bond dipoles add up instead of canceling, and that little is why water dissolves , why ice floats, and why a microwave can heat your soup.