Which of the following statement is not correct from the view point of molecular orbital theory?

Options

A. `Be_(2)` is not a stable molecule.
B. `He_(2)` is not stable but `He_(2)^(+)` is expected to exist.
C. Bond strength of `N_(2)` is maximum amongst the homonuclear diatomic molecules belonging to the second period.
D. The order of energies of molecule orbitals in `N_(2)` molecule is
`sigma2s lt sigma^(star)2s lt sigma2p_(z) lt pi2p_(x)=pi2p_(y) lt pi^(star)2p_(x)=pi^(star)2p_(y) lt sigma^(star)2p_(z)`

Answer

D

Explanation

d) Existance of molecule, bonding nature and energy order of molecule orbitals can be explained on the basis of molecular orbital theory as follows.
i) Molecules having zero bond order never exists while molecular having non-zero bond order is either exists or expected to exist.
ii) Higher the value of bond order, higher will be its bond strength.
Electrons present in bonding molecular orbital are known as bonding electrons. `N_(6)` and electrons present on anti-bonding molecular orbital are known as anti-bonding electrons `(N_(a))` and half of their difference is known as bond order i.e.
a) `Be_(2) (4+4=8)=sigma1s^(2), sigma^(star)1s^(2), sigma1s^(2),sigma^(star)2s^(2)`
Bond order (BO)=`1/2`
[Number of bonding electrons`(N_(b))` - Number of anti-bonding electrons `N_(a)`]
Here, bond order of `Be_(2)` is zero, Thus, it does not exist.
b) `He_(2)(2+2=4)=sigma1s^(2), sigma^(star)s^(2)`
`BO=(2-2)/(2) = 0`
Here, bond order of `Be_(2)` is zero, thus, it does not exist.
`He_(2)^(+)(2+2-1=3)=sigma1s^(2), sigma^(star)1s^(-1)`
`BO=(2-1)/2=0.5`
Since, the bond order is not zero, this molecules is expected to exist.
c) `N_(2)(7+7=14)=sigma1s^(2), sigma^(star)1s^(2), sigma2s^(2), pi2p_(X)^(2)=pi2p_(Y)^(2), sigma2p_(z)^(2)`
`BO=(10-4)/2=3`
Thus, dinitrogen `(N_(2))` molecule contain triple bond no any molecule of second product have more than doubled bond. Hence, bond strength of `N_(2)` is maximum amongst the homonuclear diatomic molecules belonging to the second period.
d) It is incorrect. The correct order of energies of molecular orbitals in `N_(2)` molecule is
`sigma2s lt sigma^(star)2s lt pi2p_(x)=pi2p_(y) lt sigma2p_(z) lt pi^(star)2p_(x)=pi^(star)2p_(Y) lt sigma^(star)2p_(z)`