We have seen that 1*s* atomic orbitals on two atoms form two molecular orbitals designated as σ1*s* and σ*1*s*. In the same manner, the 2*s* and 2*p* atomic orbitals (eight atomic orbitals on two atoms) give rise to the following eight molecular orbitals:

Antibonding MOs σ **2s* σ**2p* _{z}*π ***2p* _{x}*π ***2p*_{y}

Bonding MOs σ*2s* σ*2p*_{z}*π **2p* _{x}*π **2p*_{y }

The energy levels of these molecular orbitals have been determined experimentally from spectroscopic data for homonuclear diatomic molecules of second row elements of the periodic table. The increasing order of energies of various molecular orbitals for O_{2} and F_{2} is given below:

*σ**1s *< *σ** * 1s *< *σ** 2 s *< *σ** * 2 s *< *σ** 2 p _{z}* <

*(*

*π*

*2 p*=

_{x}*π*

*2 p*

_{y}) < (*π*

** 2 p*=

_{x}*π*

** 2 p*<

_{y})*σ*

** 2 p*

_{z}

However, this sequence of energy levels of molecular orbitals is not correct for the remaining molecules Li_{2}, Be_{2}, B_{2}, C_{2}, N_{2}. For instance, it has been observed experimentally that for molecules such as B_{2}, C_{2}, N_{2}, etc. the increasing order of energies of various molecular orbitals is

*σ**1s *< *σ** * 1s *< *σ** 2s *< *σ** * 2s *<* ( **π** 2 p ** _{x}* =

*π*

*2 p*

_{y}*)*

*< σ*

*2 p*

_{z}*<*

*(*

*π*

** 2 p*

_{x}*=*

*π*

** 2 p*

_{y}*)*<

*σ*

** 2 p*

_{z}The important characteristic feature of this order is that the energy of **σ** *2 p* * _{z}* molecular orbital is higher than that of

**π**

*2p*

*and*

_{x}**π**

*2p*

*molecular orbitals.*

_{y}