Combination of 2s atomic orbitals
Like 1s-orbitals, 2s-orbitals combine by addition and subtraction of overlappingwavefunctions to form bonding and antibonding molecular orbitals. These are labeled as s2s,and s*2s. These molecular orbitals have exactly the same shapes as s1s and s*1s MOs but they are slightly larger in size.
Combination of 2p atomic orbitals
There are three p-orbitals namely 2px 2py and 2pz, which are directed in space along the x, y and z, coordinate axes respectively. By convention, z-axis is assumed to be the internuclear axis. The x and y-axes would then be perpendicular to the internuclear axis. The combination of 2p-orbitals forms two types of molecular orbitals; sigma (s) MOs and pi (p) MOs as discussed below:
(i) Formation of s - molecular orbitals
When the two 2pz orbitals are brought closer along the internuclear axis, they overlap head on forming a-bond. When the two orbitals overlap by the addition of electron waves (when + + lobes overlap), bonding molecular orbital is formed. This may be designated as s2pz. On the other hand, when the two wave functions are subtracted, an antibonding molecular orbital is formed. This is designated as s*2pz. Thus, two 2pz, atomic orbitals combine to form two molecular orbitals, which are a bonding molecular orbital (s2pz,) and an antibonding molecular orbital (s*2pz).
(ii) Formation of p-molecular orbitals
When a 2px atomic orbital of one atom approaches the 2px atomic orbital of another atom, the overlapping occurs sidewise and not end-to-end. The molecular orbital formed by sidewise or lateral overlap is known as pi (p) molecular orbital. The molecular orbital formed is not symmetrical about the internuclear axis. The molecular orbital formed by overlapping of two 2px atomic orbitals by the addition of electron waves is called bonding pi molecular orbital designated as p2px while the molecular orbital formed by the overlapping of two 2px atomic orbitals by the subtraction of electron waves is called antibonding molecular orbital labelled as p*2px. These are shown in the figure given below.
fig 1.12 - Bonding and antibonding molecular orbitals from 2px atomic orbitalsSimilarly, when a 2py orbital of one atom overlaps with a 2py orbital of another atom, they also overlap sidewise forming two molecular orbitals p2py and p*2py. These are exactly similar to p2px and p*2px molecular orbitals.
