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within which the probability of finding the electron has some definite value, say 90%. The molecular orbital designated with an " * " is the unstable antibonding state, otherwise, it is the stable bonding state. The + or - sign signifies a positive or negative value for the wave function. Each orbital can accommodate two electrons with opposite spin direction. Recently (in 2005), a method has been developed to take image of a molecule by using a short laser pulse lasting just 3 x 10-14 second. Figure 12-14a shows a electron orbital of a nitrogen molecule as imaged by such technique. It agrees quite well with the orbital as calculated from theoretical models (Figure 12-14b). The colours represent the amplitude of the wave function - the |
Figure 12-14a N2 Image [view large image] |
Figure 12-14b N2 Model |
electron is most likely to be found at the red and dark blue areas. Producing a three-dimensional image requires repeating the process at different angles, like a hospital CT scanner. |
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In the H2O molecule, the electric field around the oxygen atom is stronger than that around the hydrogen. The electrons from the hydrogen atoms are drawn close to the oxygen. This leaves the hydrogen atoms positively charged at one end. The four pairs of valence electrons around the oxygen atom (six contributed by the O atom -- 2 in the 2s, 4 in the 2p states; and one each by the H atoms in the 1s state) occupy four sp3 orbitals that form a tetrahedral pattern. The energy levels for these four pairs of electrons are lower than the original levels in separated atoms as shown in Figure 12-15a (MO1 - MO4). Since the positively charged atomic nucleus for |
Figure 12-15a H2O Energy Levels [view large image] |
Figure 12-15b Hydrogen Bond [view large image] |
the hydrogen is partially exposed, it often attracts to other negatively charged orbitals such as the electron pairs of the oxygen atom in another H2O molecule (see Figure 12-15b). |
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This is called hydrogen bond. It is different from the covalent bond since there is no orbital overlap; it is not an ionic bond since there is no charge transfer from one atom to another. The strength of the hydrogen bond is about 10 times weaker than the covalent and ionic bonds. Hydrogen bonds are important in fixing properties such as solubilities, melting points, and boiling points, and in determining the form and stability of crystalline structures. Molecules such as water carrying hydrogen bonds are called polar molecules. They play a crucial role in biological systems. |
Figure 12-16 Water |