Molecular orbital

Molecular orbital

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Relationship between Electronic configuration and Molecular behaviour :

1)  Bond order :

 It is defined as the number of covalent bonds between the two combining atoms of a molecule.

Bond order = 0.5 (Nb – Na)

Nb = number of bonding electrons or number of electrons in bonding M.O’s

Na = number of antibonding electrons

2)  Stability of molecule :

  It is determined by bond order.Higher is the bond order greater is the stability of molecule.

i) If Nb > Na, then molecule will be stable.

ii) If Nb=Na, then formation of the molecule will not take place.

iii) If Nb< Na, then molecule will be unstable.

3)  Magnetic property :

 A molecule in which all the electrons are paired, is called diamagnetic while molecule which has one or more unpaired electron is called paramagnetic.

Molecular orbital diagram of H2 (Hydrogen molecule) :

Number of electrons in H2 = 2

Electronic configuration of H2 = σ1s2

Bond order = 0.5 (Nb-Na)



B.O = 0.5 (2-0)

B.O= 1

M.O. diagram of He2 molecule :

2He = 1s2

Electronic configuration of He2 = σ1s2, σ*1s2

Nb =2, Na=2

B.O =0.5(0-0)

B.O=0 (Zero)

Molecules having zero bond order do not exist.

Molecular orbital diagram of H2+ (Hydrogen molecule ion) :

H2+ = σ1s1

B.O =0.5 (Nb- Na)

Nb =1 , Na = 0

B.O = 0.5(1-0)

B.O =0.5


Molecular orbital diagram of Li2 & Be2 :

Number of electrons in Li2 molecule =6

Li2 = σ1s2,σ*1s2,σ2s2

Nb=4, Na=2

B.O =0.5(Nb- Na)

B.O = 0.5 (4-2)

B.O = 1

Number of electrons in Be2 molecule = 8

Be2 = σ1s2,σ*1s2,σ2s2,σ*2s2 

Nb=4, Na=4

B.O =0.5(Nb- Na)

B.O = 0.5 (4 – 4)

B.O = 0

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Molecular orbital  diagram of C2 molecule :

Number of electrons in C2 molecule = 12

C2 = σ1s2,σ*1s2,σ2s2,σ*2s2, π2py2= π2pz2 ,σ2px0

Nb=8, Na=4

B.O =0.5(Nb- Na)

B.O = 0.5 (8-4)

B.O =2

Molecular orbital diagram of N2 molecule :

7N = 1s2,2s2,2p3

Number of electrons in N2 molecule = 14

N2 = σ1s2,σ*1s2,σ2s2,σ*2s2,π2py2 = π2pz2,σ2px2,

 Nb=10, Na=4

B.O =0.5(Nb- Na)

B.O = 0.5 (10-4)

B.O =3

Molecular orbital diagram of O2 molecule :

8O – 1s2 ,2s2,2p4

Number of electrons in O2 molecule = 16

Electronic configuration of O2:

 σ1s2,σ*1s2,σ2s2,σ*2s2,σ2px2,π2py2= π2pz2,π*2py1=π*2pz1,σ*2px0.

Nb= 10

Na =6

B.O = 0.5 (10-6)=2

O2 molecule is paramagnetic because  two  unpaired electrons are present. One unpaired  in π*2py1 and one in π*2pz1.

Molecular Orbital diagram of  O2 ion :

This is superoxide ion. It has 8+8+1=17 electrons. The M.O configuration is O2 is

σ1s2,σ*1s2,σ2s2,σ*2s2,σ2px2,π*2py2= π2pz2,π*2py2=π*2pz1,σ*2px0.



B.O =0.5 (Nb-Na)



Molecular Orbital diagram of O2 ion :

This is peroxide ion. It has 8+8+2 =18 electrons. The M.O configuration is:

O2– –— = σ1s2,σ*1s2,σ2s2,σ*2s2,σ2px2, π2py2= π2pz2,π*2py2=π*2pz2,σ*2px0.



B.O =0.5 (Nb-Na)


B.O= 1

Molecular Orbital diagram of Hetero nuclear diatomic molecule :

Molecular Orbital diagram of Carbon monoxide molecule (CO):

Total electrons:6+8 =14

CO =σ1s2,σ*1s2,σ2s2,σ*2s2, σ2px2, π2py2= π2pz2.



B.O =0.5 (Nb-Na)


B.O= 3

Molecular Orbital diagram of NO(nitric oxide)molecule :

Electronic configuration of NO: σ1s2,σ*1s2,σ2s2,σ*2s2,σ2px2,π2py2= π2pz2, π*2py1=π*2pz0,

Number of electrons =7+8 =15



B.O =0.5 (Nb-Na)


B.O =2.5