Question

Give reason why H2+ ions are more stable than H2- though they have the samebond order.

Answer 1

because H2- contains one electron in the antibonding orbital which results in repulsion and decrease the stability . on the other hand H2+ does not contain any electron in the antibonding molecular orbital.

Answer 2

Answer : $H_2^+$ ion will be more stable as compared to $H_2^-$ ion due to absence of anti-bonding electrons.

Explanation :

According to the molecular orbital theory, the general molecular orbital configuration up to 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 1 electron present in hydrogen.

(a) The number of electrons present in $H_2^+$ molecule = 2(1) - 1 = 1

The molecular orbital configuration of $H_2^+$ molecule will be,

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

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

The bonding order of $H_2^+$ = $\frac{1}{2}\times (1-0)=0.5$

(b) The number of electrons present in $H_2^-$ molecule = 2(1) + 1 = 3

The molecular orbital configuration of $H_2^-$ 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 bonding order of $H_2^-$ = $\frac{1}{2}\times (2-1)=0.5$

From this we conclude that the both the molecule have same bond order. But as we know that the more the number of electrons are in anti-bonding, the molecule will be less stable. That means anti-bonding decreases the stability of the molecule.

In $H_2^-$, the more number of electrons present in anti-bonding orbital but in $H_2^+$, there is no electrons present in anti-bonding orbital. So, $H_2^+$ ion will be more stable as compared to $H_2^-$ ion.