The Hydrosphere
The electronic configuration of atomic oxygen is 1s2 2s2 2p4 and of atomic hydrogen is 1s1. In the water molecule the three nuclei are surrounded by ten electrons; the two 1s electrons of oxygen are confined to the vicinity of the oxygen nucleus, and the
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Structure and Properties of Water The Water Molecule
The electronic configuration of atomic oxygen is 1s2 2s2 2p4 and of atomic hydrogen is 1s1• In the water molecule the three nuclei are surrounded by ten electrons; the two 1s electrons of oxygen are confined to the vicinity of the oxygen nucleus, and the other eight electrons are in four approximately sp3 hybrid orbitals which point to the corners of a distorted tetrahedron (Fig. 1). Two of the orbitals are bonding (contain bonding-pair electrons) and are directed along the 0-H bond axes, and the two of the orbitals are nonbonding (contain lone-pair electrons) and are directed above and below the H-0-H molecular plane. The planar H-0-H bond angle in the isolated water
b:
hybrid orbital with bonding pair electrons
1 : hybrid orbital with lone pair electrons Fig. 1. The structure of the water molecule. The two hydrogen atoms and the two electron lone-pairs form the apices of a distorted tetrahedron, at the center of which is oxygen. (After [1])
O. Hutzinger, The Natural Environment and the Biogeochemical Cycles © Springer-Verlag Berlin Heidelberg 1980
J. Westall, W. Stumm
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SYMMETRIC STRETCHING
u1
BENDING
u2
ASSYMETRIC STRETCHING
u3
Fig. 2. The normal modes of vibration of the isolated water molecule. The principle IR frequencies associated with these vibrations are given in Table 1.
molecule is 104.5 °, compared to 109.5 o for a regular tetrahedron; the 0-H bond length in the isolated water molecule is 0.096 nm. The lone-pair electrons probably contribute an insignificant amount to the permanent dipole moment, but a significant amount to the induced dipole moment. The isolated water molecule has C2v symmetry; it has a two fold axis of rotational symmetry, C2, the line bisecting the H-0-H angle, and a plane of reflection v passing through the axis and normal to the plane of the molecule. The water molecule has three normal modes of vibration (Fig. 2), the symmetric stretch, bending, and the assymmetric stretch. The prominent infrared frequencies associated with these normal modes are given in Table 1 for the isolated water molecule.
Table 1. Vibrational frl!quencies of an isolated H 20 molecule [1] Transition between ground state and upper state with quantum numbers a VI
v2
v3
0 1 0 0 0 0 1 1 2 0
1 0 0 2 1 2 0 1 0 0
0 0 1 0 1 1 1 1 1 3
a
v1: symmetric streching v2: bending v3: assymrnetric streching
Absorption frequency (cm- 1)
1,594.59 3,656.65 3,755.79 3,151.4 5,332.0 6,874 7,251.6 8,807.05 10,613.12 11,032.36
The Hydrosphere
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Hydrogen Bonding
Hydrogen bonding in water is the specific association of the hydrogen atom of one molecule with the lone pair electrons for another. While there is ample evidence of hydrogen bonding in ice and liquid water, little direct evidence of hydrogen bonding in water vapor exists. The energy associated with hydrogen bonds may be defined and calculated in different ways; values range from 5.4 to 18.8 kJ ·mol-t H-bond in liquid water and from 17.8 to 32.2 kJ · mol- 1 H-bond in ice [1]. The 0-H bond i
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