The water molecule has two
hydrogen atoms held at an angle of 104° 27' and at distances of
nearly 0.1 nm from the oxygen atom. The hydrogen atoms are held
at about 0.1 nm from each other. The molecule is covalently bonded,
electrons being shared between the hydrogens and the oxygen, giving
overall electrical neutrality, but the oxygen nucleus has greater
affinity for electrons than that of the hydrogen. There is thus,
on average, a slight displacement of negative charge towards the
oxygen, which leaves a slight positive charge on the hydrogen atoms.
Consequently there is an attraction between the slightly positive
hydrogen on one molecule, and the slightly negative oxygen on the
other, which links them together with a 'hydrogen bond'. The angles
at which oxygen and hydrogen are held in the water molecule, coupled
with this hydrogen bonding, result in a crystal structure for ice
that is based on tetrahedra.
Oxygen is at the centre of each tetrahedron, surrounded by four
hydrogen atoms, two covalently and two hydrogen-bonded. Such a crystal
is quite open, compared with those of other substances, which often
have twelve neighbours (as opposed to four in ice) packed around
each molecule, Ice consequently has a low density. It floats on
liquid water and, by forming an insulating layer at the surface
of water bodies, often prevents them from freezing solid and thus
killing fish and other organisms.
The hydrogen bonding in ice is quite strong, because the displacement
of negative charge towards the oxygen atom is powerful. The temperature
at which melting of ice takes place - a measure of the energy needed
to begin breaking down the hydrogen-bonded structure - is thus relatively
high, compared with H2S and H2Se, where the
charge displacement in the molecules is small.