Chemistry Bonding in Some Homonuclear Diatomic Molecules

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.

 
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