Water H₂O: Geometry of the Molecule

A bent molecule

The water molecule consists of one oxygen and two hydrogen atoms. The two O–H bonds are 95.8 pm long and enclose an angle of 104.5°. The molecule is bent, not straight. This shape is the cause of many of water’s special properties, for example its strong dipole moment of 1.85 D.

Four electron pairs, one tetrahedron

Oxygen brings six valence electrons and each hydrogen one. That makes eight electrons, or four electron pairs: two bonding pairs (the O–H bonds) and two lone pairs that belong to oxygen alone. Four pairs arrange themselves to repel each other as little as possible, namely at the corners of a tetrahedron with angles of 109.5°. This is the core idea of the VSEPR model (Valence Shell Electron Pair Repulsion).

The measured 104.5° lies a little below this because lone pairs need more room than bonding pairs and squeeze them together. Choose “Bonds and lone pairs” above to see exactly these four pairs as orbitals: two along the bonds and two like “rabbit ears” above the oxygen.

All orbitals are present at once

The orbitals in the picture are not variants of the water molecule. The molecule has five occupied orbitals (counting the oxygen core), each with two electrons, and all are present at the same time. We show them one by one so they can be read. Their sum is the “Total density”. The two upper orbitals (marked *) are empty.

Canonical and localized orbitals

The calculation first yields molecular orbitals that extend over the whole molecule: 2a₁, 1b₂, 3a₁ and 1b₁ are the occupied valence orbitals, plus the oxygen core orbital 1a₁. They have different energies, which is also what the photoelectron spectrum measures (12.6, 14.8, 18.7 and 32.2 eV). This does not fit the picture of two equivalent lone pairs.

The resolution: occupied orbitals can be mixed mathematically without changing the total density. The resulting localized orbitals correspond to the lines and dots of the Lewis formula. Both pictures are correct. One fits spectroscopy better, the other chemical intuition.

Why isn’t water straight?

The Walsh diagram below the picture shows how orbital energies change with the bond angle. In the linear molecule (180°) the two lone pairs 1b₁ and 3a₁ are equivalent. If the molecule bends, the energy of the 3a₁ orbital drops, because it can then mix in some oxygen 2s character and bond more strongly with the hydrogens. This is the gain that bends the molecule. The sum of the orbital energies (second graph) therefore has its minimum at an angle below 180°.

The approximation used here (extended Hückel method) does not hit the angle exactly: it gives about 120° rather than 104.5°. But it shows the correct trend and makes the principle visible, which also holds in more accurate calculations.

Why this matters for water

Polarity: oxygen attracts electrons more strongly. Because of the bend the centres of charge do not coincide. The molecule has a positive and a negative pole, see “Total density”.
Hydrogen bonds: the lone pairs of one molecule attract the hydrogens of the next. Each molecule can form up to four such bridges.
Ice: in ice the molecules form an open tetrahedral network. That is why ice is lighter than liquid water and floats.
Solvent: thanks to its polarity, water dissolves salts and many organic substances.

How accurate are the pictures?

The orbitals are computed with the extended Hückel method. It uses just six basis functions (O 2s, three O 2p and two H 1s) and simple approximations for the energies. That suffices for shapes, symmetries and trends. Absolute energies and angles come from more accurate methods such as Hartree-Fock or density functional theory. The measured values (angle, bond length, ionization energies) are taken from the literature.

Think about it

What angle would a molecule like BeH₂ have without lone pairs?

180°. Just two bonding pairs sit as far apart as possible, on opposite sides. Lone pairs are the reason water bends.

Why does the energy of the 1b₁ orbital not change when you vary the angle?

It is a pure oxygen 2p orbital perpendicular to the molecular plane. The hydrogen atoms lie in its nodal plane, so it does not feel any change in the H–O–H geometry.

Onward to a larger molecule: benzene. Back: hydrogen atom.

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Last updated: 2026-10-03