question_answer 38)

Write the important conditions required for the linear combination of atomic orbitals to form molecular orbitals.

Answer:

The formation of molecular orbitals by linear combination of atomic orbitals can take place only if the following conditions are fulfilled: 1. The combining atomic orbitals must have same or nearly same energy, i.e.., comparable energy. For homonuclear atomic molecule, \[{{A}_{2}}\]\[1s\] atomic orbital of one atom can combine with 1s atomic orbital of another atom or 2s can combine with 2s, 2p with 2p and so on. Is cannot combine with 2s or 2s cannot combine with 2p as the energy difference is appreciably high. However, such combinations can take place for heteronuclear diatomic molecules, AB, if the energy difference is not very high. 2. The combining atomic orbitals must have proper orientation, i.e. same symmetry about the molecular axis. For example, taking z-axis as the molecular axis, \[2{{p}_{z}}\] -orbital of one atom can overlap with \[2{{p}_{z}}\] -orbital of another atom but not with \[2{{p}_{x}}\] - or \[2{{p}_{y}}\] -orbitals because of their different symmetries. Similarly 2s-orbital of one atom can overlap with \[2{{p}_{z}}\] -orbital but not with \[2{{p}_{x}}\] - or \[2{{p}_{y}}\] -orbitals of another atom on account of improper orientation [Fig. 4.5 (a) and (b)]. 3. The combining atomic orbitals must overlap to the maximum extent. Greater the extent of overlap, higher will be the electron density between the nuclei of a molecular orbital.