Question

explain why He2 does not exist under ordinary condition ? calculate bond order of He2+

Answer 1

It has equal number of electrons in the bonding and anti bonding electrons (From Molecular Orbital Theory), thus making the net bond order = (Bonding electrons/2)-(Anti electrons/2) = 0.

And no molecule can exist with 0 bonds.

Answer 2

Explanation :

According to the molecular orbital theory, the general molecular orbital configuration upto nitrogen will be,

$(\sigma_{1s}),(\sigma_{1s}^*),(\sigma_{2s}),(\sigma_{2s}^*),(\sigma_{2p_z}),[(\pi_{2p_x})=(\pi_{2p_y})],[(\pi_{2p_x}^*)=(\pi_{2p_y}^*)],(\sigma_{2p_z}^*)$

As there are 2 electrons present in helium.

The number of electrons present in $He_2$ = 2(2) = 4

The molecular orbital configuration of $He_2$ molecule will be,

$(\sigma_{1s})^2,(\sigma_{1s}^*)^2,(\sigma_{2s})^0,(\sigma_{2s}^*)^0,(\sigma_{2p_z})^0,[(\pi_{2p_x})^0=(\pi_{2p_y})^0],[(\pi_{2p_x}^*)^0=(\pi_{2p_y}^*)^0],(\sigma_{2p_z}^*)^0$

The formula of bond order = $\frac{1}{2}\times (\text{Number of bonding electrons}-\text{Number of anti-bonding electrons})$

The bond order of $He_2$ = $\frac{1}{2}\times (2-2)=0$

As, the bond order of $He_2$ is zero that means there is no bond present between the He-He atom. So, $He_2$ does not exist under ordinary condition.

The number of electrons present in $He_2^+$ molecule = 2(2) - 1 = 3

The molecular orbital configuration of $He_2^+$ molecule will be,

$(\sigma_{1s})^2,(\sigma_{1s}^*)^1,(\sigma_{2s})^0,(\sigma_{2s}^*)^0,(\sigma_{2p_z})^0,[(\pi_{2p_x})^0=(\pi_{2p_y})^0],[(\pi_{2p_x}^*)^0=(\pi_{2p_y}^*)^0],(\sigma_{2p_z}^*)^0$

The bond order of $He_2^+$ = $\frac{1}{2}\times (2-1)=0.5$

Therefore, the bond order of $He_2^+$ is, 0.5