Question

why does He2+ exist whereas He2 does not? huckel theory

Answer 1

Let me answer this using this concept of Bond Order based on the Huckel Molecular Orbital Theory.

The molecular orbital configuration is based on the number of electrons. The number of electrons in  $He_{2}$ is 4.
Hence, the molecular orbital configuration of  $He_{2}$ is:
σ $1s^{2}$ σ* $1s^{2}$ 

Bond order is given by:
[ ( No . of electrons in Bonding MO ) - ( No . of electrons in Anti Bonding MO ) ] / 2

Thus, bond order of  $He_{2}$ is (2-2)/2 = 0/2 = 0
When bond order is 0, such a molecule does not exist.

Now, coming to $He_{2}^{+}$, the number of electrons are 4-1 = 3 (+ indicates that it is deficient of 1 electron)

Hence, the molecular orbital configuration is:
σ $1s^{2}$ σ* $1s^{1}$ 

The bond order will be (2-1)/2 = 1/2
The bond order is not equal to 0, hence this molecule exists.

Therefore, $He_{2}^{+}$ exists while $He_{2}$ does not exist.