Question

Based on Molecular Orbital Theory, rank C2–, C2, and C2+ in order of increasing bond order given that C2 has the valence MO energy sequence:

σ2s < σ*2s < π2py = π2pz < σ2px < π*2py = π*2pz < σ*2px

(1) C2– < C2 < C2+

(2) C2– < C2+ < C2

(3) C2+ < C2 < C2–

(4) C2 < C2+ < C2–

(5) C2+ < C2– < C2

Answer 1

σ and σ* orbital can have maximum of 2 electrons

π and π* orbital can have maximum of 4 electrons

order of filling of orbitals is:

σ1s σ*1s σ2s σ*2s π2p σ2p π*2p σ*2p

for C2-:

1 C atom have 6 electrons

Here, number of C is 2

so, total number of electrons in C2 is 12

Since charge is -1

Total number of electron = 13

So, total number of electrons to fill is 13

Lets fill the orbital

the filled orbital is σ1s2 σ*1s2 σ2s2 σ*2s2 π2p4 σ2p1

Number of electron in bonding orbital = 9

Number of electron in Anti bonding orbital = 4

Bond Order = 0.5*(Number of electron in bonding orbital - Number of electron in Anti bonding orbital)

= 0.5*(9 - 4)

= 2.5

for C2:

1 C atom have 6 electrons

Here, number of C is 2

so, total number of electrons in C2 is 12

So, total number of electrons to fill is 12

Lets fill the orbital

the filled orbital is σ1s2 σ*1s2 σ2s2 σ*2s2 π2p4

Number of electron in bonding orbital = 8

Number of electron in Anti bonding orbital = 4

Bond Order = 0.5*(Number of electron in bonding orbital - Number of electron in Anti bonding orbital)

= 0.5*(8 - 4)

= 2

FOR C2+

1 C atom have 6 electrons

Here, number of C is 2

so, total number of electrons in C2 is 12

Since charge is +1

Total number of electron = 11

So, total number of electrons to fill is 11

Lets fill the orbital

the filled orbital is σ1s2 σ*1s2 σ2s2 σ*2s2 π2p3

Number of electron in bonding orbital = 7

Number of electron in Anti bonding orbital = 4

Bond Order = 0.5*(Number of electron in bonding orbital - Number of electron in Anti bonding orbital)

= 0.5*(7 - 4)

= 1.5

Answer: (3)