Chemistry Bonding in Some Homonuclear Diatomic Molecules
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### Topics Covered :

● Bonding in Some Homonuclear Diatomic Molecules
● Hydrogen Molecule (H_2)
● Helium Molecule (He_2)
● Lithium Molecule (Li_2)
● Carbon Molecule (C_2)
● Oxygen Molecule (O_2)

### Bonding in Some Homonuclear Diatomic Molecules :

In this section we shall discuss bonding in some homonuclear diatomic molecules.

### Hydrogen molecule (H_2 ) :

=> It is formed by the combination of two hydrogen atoms.

=> Each hydrogen atom has one electron in 1s orbital.

● Therefore, in all there are two electrons in hydrogen molecule which are present in σ1s molecular orbital.

● So electronic configuration of hydrogen molecule is

H_2 : ( sigma 1 s)^2

=> The bond order of H_2 molecule can be calculated as given below :

Bond order  = (N_b - N_a)/2 = (2-0)/2 =1

● This means that the two hydrogen atoms are bonded together by a single covalent bond.

=> The bond dissociation energy of hydrogen molecule is 438 kJ mol^(–1) and bond length equal to 74 pm.

=> Since no unpaired electron is present in hydrogen molecule, therefore, it is diamagnetic.

### Helium molecule (He_2 ) :

=> The electronic configuration of helium atom is 1s^2.

● Each helium atom contains 2 electrons, therefore, in He_2 molecule there would be 4 electrons.

=> These electrons will be accommodated in σ1s and σ^ast1s molecular orbitals leading to electronic configuration :

He_2 : (sigma 1 s)^2 ( sigma^(**) 1 s)^2

=> Bond order of He_2 is ½(2 – 2) = 0

● Therefore, He_2 molecule is unstable and does not exist.

=> Similarly, it can be shown that Be_2 molecule (σ1s)^2 (σ**1s)^2 (σ2s)^2 (σ**2s)^2 also does not exist.

### Lithium molecule (Li_2 ) :

=> The electronic configuration of lithium is 1s^2, 2s^1.

=> There are six electrons in Li_2.

● The electronic configuration of Li_2 molecule, therefore, is

Li_2 : (σ1s)^2 (σ**1s)^2 (σ2s)^2

● The above configuration is also written as KK(σ2s)^2 where KK represents the closed K shell structure (σ1s)^2 (σ**1s)^2.

● From the electronic configuration, there are four electrons present in bonding molecular orbitals and two electrons present in antibonding molecular orbitals.

=> Bond order = ½ (4 – 2) = 1.

● It means that Li_2 molecule is stable.

=> Since it has no unpaired electrons it should be diamagnetic.

● Diamagnetic Li_2 molecules are known to exist in the vapour phase.

### Carbon molecule (C_2 ) :

=> The electronic configuration of carbon is 1s^2 2s^2 2p^2.

=> There are twelve electrons in C_2.

● The electronic configuration of C_2 molecule, therefore, is

C_2 : ( sigma 1 s)^2 ( sigma** 1 s)^2 ( sigma 2s)^2 (sigma** 2s)^2 (pi 2p_x^2 equiv pi 2p_y^2)

or KK ( sigma 2s)^2 (sigma** 2s)^2 (pi 2p_x^2 equiv pi 2p_y^2)

=> The bond order = ½ (8 – 4) = 2.

=> C_2 should be diamagnetic.

● Diamagnetic C_2 molecules have indeed been detected in vapour phase.

text(Note :) Double bond in C_2 consists of both pi bonds because of the presence of four electrons in two pi molecular orbitals.

● In most of the other molecules a double bond is made up of a sigma bond and a pi bond.

=> In a similar fashion the bonding in N_2 molecule can be discussed.

### Oxygen molecule (O_2 ) :

=> The electronic configuration of oxygen atom is 1s^2 2s^2 2p^4.

=> Each oxygen atom has 8 electrons, hence, in O_2 molecule there are 16 electrons.

● The electronic configuration of O_2 molecule, therefore, is

O_2 : (sigma 1 s)^2 ( sigma** 1 s)^2 ( sigma 2s)^2 (sigma** 2s)^2 (sigma 2p_z)^2 (pi 2p_x^2 equiv pi 2p_y^2) (pi** 2p_x^2 equiv pi** 2p_y^2)

or O_2 : (KK) (sigma 2s)^2 (sigma** 2s)^2 (sigma 2p_z)^2 (pi 2p_x^2 equiv pi 2p_y^2) (pi** 2p_x^2 equiv pi** 2p_y^2)

● From the electronic configuration of O_2 molecule, there are ten electrons in bonding molecular orbitals and six electrons in antibonding molecular orbitals.

=> Bond order = (N_b - N_a)/2 = (10-6)/2 = 2

● So in oxygen molecule, atoms are held by a double bond.

=> It contains two unpaired electrons in π **2p_x and π **2p_y molecular orbitals, therefore, O_2 molecule should be paramagnetic, a prediction that corresponds to experimental observation.

● In this way, the theory successfully explains the paramagnetic nature of oxygen.

=> Similarly, the electronic configurations of other homonuclear diatomic molecules of the second row of the periodic table can be written.

=> In Fig.4.21 are given the molecular orbital occupancy and molecular properties for B_2 through Ne_2.

● The sequence of MOs and their electron population are shown.

● The bond energy, bond length, bond order, magnetic properties and valence electron configuration appear below the orbital diagrams.