CBSE Class 11-science Answered

For LCAO, the two atomic orbitals must be of similar energy and the number of molecular orbitals formed should be equal to the number of atomic orbitals taking part in combination.

An LCAO approximation is a quantum superposition of atomic orbitals, used to calculate molecular orbitals in quantum chemistry.

According to Linear combination of atomic orbitals (LCAO) approach, the molecular orbital formed by additive effect of two atomic orbitals are called as bonding orbitals(N_B) and that of due to subtractive effect of atomic orbitals is called as antibonding molecular orbitals (N_A).

Now, the bond order of the molecule is calculated as: Bond order = 1/2 (N_B -N_A )

Only that molecule will be stable or constructive for which, N_B >N_A ( i.e bond order greater than zero) and if N_B < N_A or N_B =N_A , i.e bond order zero or negative then the molecule will not exist.

In case of He₂ molecule, the electronic configuration of He = 1s²

so, the energy level MO diagram will be as :

Here, the bond order = 1/2 (N_B -N_A) = 1/2 (0-0) = 0

Since, bond order is zero, so the molecule will not exist.