The bent shape is water's normal resting state
For a free water molecule in the gas phase, experiments give an H–O–H bond angle of 104.52 ± 0.05° and an O–H bond length of 0.9572 ± 0.0003 Å [1][2]. That angle is far from the 180° a straight molecule would have. Calculations based on the spectroscopic potential energy surface—essentially a map of the molecule's energy at different geometries—find an equilibrium angle of 104.501 ± 0.005°, essentially the same value [1][2]. So the bend is not a fleeting accident; it is the shape the molecule settles into when left alone.
The same bent geometry persists when water molecules are packed together. In liquid water, neutron diffraction experiments show the angle widening only slightly, to 106.1 ± 1.8°, with the bond length growing to 0.970 ± 0.005 Å [1][2]. In most ice structures, bond angles are close to the tetrahedral angle, though some ice structures deviate noticeably [1][2]. The molecule remains bent across these environments; what changes is the precise angle, not the basic non-linear arrangement.
Why the hydrogens do not line up opposite each other
The supplied papers establish the bent geometry and measure how it varies, but they do not spell out the underlying electron-level reason in their abstracts. What the evidence does show is that the bent shape is the energetically preferred equilibrium: high-level ab initio calculations—computer simulations of the molecule's electrons—predict this geometry as the low-energy structure [1][2]. In other words, the molecule is not straight because a straight arrangement is not the arrangement its electrons settle into.
The angle is also not rigidly fixed. The same calculations show that changing the bond angle between about 96.4° and 112.8° costs very little energy—within roughly ±1 kcal mol⁻¹—and bond lengths can similarly vary from about 0.930 Å to 0.989 Å at low cost [1][2]. This flexibility helps explain why water's angle differs between gas, liquid, and ice: the molecule can bend a little more or less without paying a large energy penalty.
Why reported angles sometimes look wildly different
If you browse crystal structures, you may see water bond angles that seem impossible. In neutron diffraction structures with a good fit quality (R factor ≤ 0.05), angles range from 100.74° to 113.92° and bond lengths from 0.91 Å to 0.99 Å [1][2]. But X-ray-derived structures show angles from 13.27° all the way to 180.00° [1][2]. That enormous spread does not mean water sometimes becomes straight; the authors conclude that at least 15% of X-ray-solved structures contain questionable water geometries [1][2]. The takeaway is that the measurement method matters: neutron results cluster near the bent equilibrium, while some X-ray results are unreliable for locating hydrogen atoms precisely.
Water's angle can also change under extreme conditions. When isolated water molecules are hit by an intense X-ray pulse and become doubly ionized (H₂O²⁺), the molecule undergoes asymmetric deformation and bond-angle opening within a few femtoseconds before fragmenting [5]. That is a transient, high-energy event, not the molecule's ordinary resting shape, but it shows that the angle is a dynamic quantity that can be pushed open when the molecule is strongly disturbed.
Sources used in this answer
How flexible is the water molecule structure? Cambridge Structural Database and ab initio calculations study.
Water's gas-phase H–O–H bond angle is 104.52 ± 0.05° with an O–H length of 0.9572 ± 0.0003 Å, and the angle shifts to 106.1 ± 1.8° in liquid water and near tetrahedral in most ices, while low-energy calculations allow the angle to vary from 96.4° to 112.8°.
How flexible is the water molecule structure? Cambridge Structural Database and <i>ab initio</i> calculations study
Experimental and computational data show water's bent geometry with a gas-phase angle near 104.5°, a liquid-phase angle near 106.1°, and low-energy ab initio flexibility across a wide range of angles and bond lengths.
Water Molecule Dispositions in the Cesium Sulfate α- and β-Alums: Single-Crystal Neutron Diffraction Studies of CsM(SO4)2.12H2O (M=V, Rh)
Neutron diffraction studies of water molecules in cesium sulfate alums find significant differences in the H–O–H bond angle depending on the molecule's coordination environment.
A configuration interaction study of the water molecule and interpretation of the walsh diagram
Configuration interaction calculations on water predict an equilibrium inter-bond angle, addressing the molecule's geometry without clearly explaining the non-linear shape.
Inner-Shell-Ionization-Induced Femtosecond Structural Dynamics of Water Molecules Imaged at an X-Ray Free-Electron Laser
After inner-shell ionization, isolated water molecules show asymmetric deformation and bond-angle opening on a femtosecond timescale before fragmenting.