Question

(i) Write the important conditions required for the linear combination of atomic
orbitals to form molecular orbitals. (2 mark)
(ii) Use molecular orbital theory to explain why the Be2 molecule does not exist.
(2 mark)
(iii) Arrange in increasing order of stability: O2, O2+, O2 , 022-. (1 mark)​

Answer 1

1)=>...

(a) The energy of combining atomic orbitals should be approximately the same.

Thus, in case of homonuclear diatomic molecule, 1s orbital of an atom can combine with 1s orbital of second atom but not with 2s orbital of second atom.

(b) To ensure maximum overlap, the combining atomic orbitals should have proper orientation.

(c) The extent of overlap should be large.

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2)=>

Be

2

molecule has MO electronic configuration KK(σs)

2

(σ

∗

2s)

2

.

The bond order is

2

2−2

=0.

Zero bond order indicates that Be

2

molecule does not exists.

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3)=>

According to MOT bond order is given by the formula:

B.O.= 2N B −N A

where NA ,N B

are the number of electrons in antibonding and bonding molecular orbital respectively,

Bond order for O 2 = 2 10−6

=2

Bond order for O 2+

= 2

10−5

=2.5

Bond order for O 2−

= 2

10−7

=1.5

Bond order for O 22−

= 2

10−8

=1

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