Molecular Orbitals of H2
Skills to develop
Describe the hydrogen molecule in light of the following:
Isn't it pretty!
|The hydrogen atom has a valency of 1 and it tends to form a chemical bond with another atom. It forms a covalent bond with another atom of the same kind to form H-H.|
|When people realized that the hydrogen atom has one electron, and that He has two electrons, they thought that a atom has a tendency to share electrons to achieve a closed shell as those of inert gases. Thus, its structure should be represented by H:H, the Lewis dot structure.|
|The invention of quantum mechanics to describe the electron configuration of atoms led to the electron configuration of 1s1 for hydrogen. This electron tends to find a partner, and sharing an electron with another hydrogen atom seems to make it more stable.|
|Valence bond (VB) theory suggested 1s atomic orbitals of two H atoms overlap to form a chemical bond.|
|Molecular orbital (MO) theory suggested atomic orbitals which not only overlap, but are simultaneously transformed into molecular orbitals with new energies and new electron distributions.|
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).
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:
Each orbital accommodates two electrons, and the two electrons in H-H filles the s1s molecular orbital (MO). Obviously, as a result of the formation of H2 molecule, the energy of the system is lowered and become more stable.
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.
After the introduction of the molecular orbital theory for the hydrogen molecule, you might want to learn more about molecular orbital theory of diatomic molecules.
Discussion: No need to memorize this value, but you have a general idea about bond length. Thus, the covalent radius for H is 37 pm.
Discussion: The simple version assumes that atomic orbitals do not change during the formation of chemical bond. They merely overlap. Modern treatments of valence bond theory do accept the fact that atomic orbitals do change a little.
If your answer is...I'm lost!
Discussion: When atoms come together to form chemical bonds, new orbitals are formed. Molecular orbitals play the same role for molecules that atomic orbitals play for atoms.
Discussion: The triple bond between C and N is represented by three pairs of electron.