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The Hydrogen Molecule

Skills to develop

The Hydrogen Molecule

Atomic hydrogen gas is unstable, unless the gas is at very high temperature. Diatomic hydrogen molecules, H2, are formed at ordinary temperature and pressure. Many models and theories have been proposed to explain the chemical bond, and here are some simplified forms of these theories, some of which may have been proposed as the society began to speculate what is happening.

Some facts are known about the hydrogen due to some careful experimental measurements. The internuclear distance is 74 pm, and the dissociation of an H-H bond into two atomic hydrogen atoms requires 7.2x10-19 J, (usually given as 435.9 kJ/mol).

Molecular Orbitals of H2

The molecular orbital approach gives further explanations on chemical bonds. When the 1s wave functions of the two H atoms are linearly combined, we get a sigma (s) bonding orbital, denoted as s1s in the diagram here. This approach is called linear combination of atomic orbitals (LCAO), in the MO approach. In this approach, the sum of the two 1s orbitals (one for each atom) is the bonding orbital. In terms of wave mechanics, the two waves constructively interact. The difference of the two orbitals forms the antibonding orbital, s1s*, due to destructive interference. It is interesting to note that the anti bonding orbital is at a higher energy than the 1s atomic orbital. The energy level can be represented below:

       -- s1s*
-- --
--
s1s Ha Ha

Each orbital accommodates two electrons, and the two electrons in H-H filles the s1s molecular orbital (MO).

The quantum mechanics of the chemical bonding is a little complicated, but the logic and concept are as simple as describe above. I have written a program for the DOS version of CAcT to plot electron densities of the s1s and s1s* electron densities, but the program cannot be executed on the web page. The excution of these type of simulations is sometimes shown during the lecture.

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