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Use molecular orbital theory to explain why the Be2 molecule does not exist but C2 can exist.

Question

Use molecular orbital theory to explain why the Be2 molecule does not exist but C2 can exist.

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Solution

Molecular Orbital Theory (MOT) is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule.

  1. According to MOT, the Be2 molecule does not exist because it has equal numbers of bonding and antibonding electrons. The total number of valence electrons in Be2 molecule is 4. When we fill these electrons in molecular orbitals, two electrons go to the bonding molecular orbital and two go to the antibonding molecular orbital. This results in zero bond order, which suggests that the Be2 molecule does not exist.

  2. On the other hand, the C2 molecule has 8 valence electrons. When we fill these electrons in molecular orbitals, four electrons go to the bonding molecular orbitals and two go to the antibonding molecular orbitals. This results in a bond order of 1, which suggests that the C2 molecule can exist.

  3. Therefore, the difference in the existence of Be2 and C2 can be explained by the Molecular Orbital Theory. The bond order calculated using MOT helps predict the stability of the molecule. A molecule with a bond order of zero is considered unstable and hence, does not exist.

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