5 Water Chemistry and Biochemistry 105theoretical calculations, and other methods. Specific
molecular dimensions from these methods differ
slightly, because they measure different properties
of water under different circumstances. However,
the O–H bond length of 95.72 pm (1 pm 10 -12m)
and the H–O–H bond angle of 104.52° have been
given after careful review of many recent studies
(Petrenko and Whitworth 1999). The atomic radii of
H and O are 120 and 150 pm, respectively. The bond
length is considerably shorter than the sum of the
atomic radii. Sketches of the water molecule are
shown in Figure 5.1 (spherical atoms assumed).
Bonding among the elements H, C, N, and O is
the key for biochemistry and life. Each carbon atom
has the ability to form four chemical bonds. Carbon
atoms can bond to other carbon atoms as well as to
N and O atoms, forming chains, branched chains,
rings, and complicated molecules. These carbon-
containing compounds are called organic com-
pounds, and they include foodstuff. The elements N
and O have one and two more electrons, respectively,
than carbon , and they have the capacity to form only
three and two chemical bonds with other atoms.
Quantum mechanicsis a theory that explains the
structure, energy, and properties of small systems
such as atoms and molecules. It provides excellent
explanations for the electrons in the atom as well as
the bonding of molecules, making the observed hard
facts appear trivial (Bockhoff 1969).
The well-known inert elements helium (He) and
neon (Ne) form no chemical bonds. The elements C,
N, and O have four, five, and six electrons more than
He, and these are called valence electrons(VE).
Quantum mechanical designation for the VE of C,
N, and O are 2s^2 2p^2 , 2s^2 2p^3 , and 2s^2 2p^4 , respective-
ly. The C, N, and O atoms share electrons with four,
three, and two hydrogen atoms, respectively. The
formation of methane (CH 4 ), ammonia (NH 3 ), and
water (H 2 O) gave the C, N, and O atoms in these
molecules eight VE. These molecules are related to
each other in terms of bonding (Fig. 5.2).
The compounds CH 4 , NH 3 , and H 2 O have zero,
one, and two lone pairs(electrons not shared with
hydrogen), respectively. Shared electron pairs form
single bonds. The shared and lone pairs dispose
themselves in space around the central atom sym-
metrically or slightly distorted when they have both
bonding and lone pairs.Figure 5.1.Some imaginative models of the water mol-
ecule, H 2 O.
Figure 5.2.Molecules of CH 4 , NH 3 , and H 2 O.