BLBS102-c05 BLBS102-Simpson March 21, 2012 12:2 Trim: 276mm X 219mm Printer Name: Yet to Come
86 Part 1: Principles/Food AnalysisFigure 5.2.Molecules of CH 4 ,NH 3 , and H 2 O.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 calledvalence
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 , respectively. 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 ,andH 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.
The lone pairs are also the negative sites of the molecule,
whereas the bonded H atoms are the positive sites. The discov-
ery of protons and electrons led to the idea of charge distribution
in molecules. Physicists and chemists call NH 3 and H 2 Opolar
moleculesbecause their centers of positive and negative charge
do not coincide. The polarizations in nitrogen and oxygen com-
pounds contribute to their important roles in biochemistry and
food chemistry. Elements C, N, and O play important and com-
plementary roles in the formation of life.
Electronegativityis the ability of an atom to attract bond-
ing electrons toward itself, and electronegativity increases in
the order of H, C, N, and O (Pauling 1960). Chemical bonds
between two atoms with different electronegativity are polar
because electrons are drawn toward the more electronegative
atoms. Thus, the polarity of the bonds increases in the order of
H C, H N, and H O, with the H atoms as the positive ends.
The directions of the bonds must also be taken into account
when the polarity of a whole molecule is considered. For exam-
ple, the four slightly polar H C bonds point toward the corner of
a regular tetrahedron, and the polarities of the bonds cancel oneanother. The symmetric CH 4 molecules do not have a net dipole
moment, and CH 4 is nonpolar. However, the asymmetric NH 3
and H 2 O molecules are polar. Furthermore, the lone electron
pairs make the NH 3 and H 2 O molecules even more polar. The
lone pairs also make the H N H, and H O H angles smaller
than the 109.5◦of methane. The chemical bonds become pro-
gressively shorter from CH 4 to H 2 O as well. These distortions
cause the dipole moments of NH 3 and H 2 O to be 4.903× 10 −^30
Cm (=1.470 D) and 6.187× 10 −^30 Cm (=1.855 D), respec-
tively. The tendency for water molecules to attract the positive
sites of other molecules is higher than that of the NH 3 molecule,
because water is the most polar of the two.
Bond lengths and angles are based on their equilibrium po-
sitions, and their values change as water molecules undergo
vibration and rotation or when they interact with each other or
with molecules of other compounds. Thus, the bond lengths,
bond angles, and dipole moments change slightly from the val-
ues given above. Temperature, pressure, and the presence of
electric and magnetic fields also affect these values.
Using atomic orbitals, valence bond theory, and molecular
orbital theory, quantum mechanics has given beautiful explana-
tions regarding the shapes, distortions, and properties of these
molecules. Philosophers and theoreticians have devoted their
lives to providing a comprehensive and artistic view of the water
molecules.WATER VAPOR CHEMISTRY AND
SPECTROSCOPYSpectroscopyis the study of the absorption, emission, or interac-
tion of electromagnetic radiation by molecules in solid, liquid,
and gaseous phases. The spectroscopic studies of vapor, in which
the H 2 O molecules are far apart from each other, reveal a wealth
of information about individual H 2 O molecules.
Electromagnetic radiation(light) is the transmission of en-
ergy through space via no medium by the oscillation of mutu-
ally perpendicular electric and magnetic fields. The oscillating
electromagnetic wavesmove in a direction perpendicular to both
fields at the speed of light (c=2.997925× 108 m/s). Max Planck