78 5 · MORE ABOUT BONDING
Example 5.7
Hydrogen bonds also exist between water molecules in liquid water and in ice.
Because each oxygen atom in the water molecule has two lone pairs of electrons,
it can attract two hydrogen atoms from other water molecules (Fig. 5.16).Comment
Hydrogen bonding has a pronounced effect on the properties of water:1.Water has a much higher melting or boiling point than compounds of similar
formula containing hydrogen. For example H 2 S, H 2 Se and H 2 Te have the same
general formulae but are all gases at room temperature. Water, of course, is a
liquid at room temperature because more energy is needed to separate H 2 O
molecules. Water molecules have much stronger attractive forces between them,
due to hydrogen bonding, than do H 2 S, H 2 Se and H 2 Te molecules.2.Water has a high surface tension. The surface of water forms a kind of ‘skin’
on which insects can walk, or a needle can float. The high surface tension of
water is due to hydrogen bonding – the intermolecular forces within the water pull
surface molecules inwards.3.A remarkable consequence of hydrogen bonding is the low density of ice. A
substance is generally more dense in the solid state than the liquid state, but ice
isless densethan water at 0°C (ice floats on water). The tetrahedral arrangement
of water molecules in ice creates a very open structure (Fig. 5.17). When ice
melts this structure breaks up and the water molecules approach each other
more closely, so that the liquid has a higher density.Hδ+Hδ+Hδ+Hδ+OδFig. 5.16Hydrogen bonding
in water. Note that the O···H
bonds are longer and weaker
than the O–H bonds.Icebergs float on water. The open arrangement of water
molecules in ice partially collapses when the substance
melts, so that the liquid has a higher density up to about
4°C.Fig. 5.17Arrangement of water molecules in ice (dotted
lines show hydrogen bonds).