Electronic configuration and Molecular Behavior:
- The distribution of electrons among various molecular orbitals is called the electronic configuration of the molecule.
Important information of configuration:
1. Stability of Molecules:
- The molecule is stable if NB is greater than NA
- The molecule is unstable if NB is less than NA
Where, NB → Bonding e- s NA → Anti-Bonding e- s
2. Bond order:
- Bond order is defined as one half the difference between the number of electrons present in the bonding and the anti-bonding orbitals.Bond order = ½ [NB - NA]
- The rules discussed in the above regarding the stability of the molecule can be restated in terms of Bond order as follows.
- A positive Bond order means a stable molecule while negative (or) zero bond order means an unstable molecule.
- Nature of Bond: Integral bond order values of 1, 2 (or) 3 correspond to single, double (or) triple bonds respectively.
3. Bond Length:
- The bond order between two atoms in a molecule may be taken as an approximate measure of the Bond length.
- Bond length decreases bond order increases.
4. Magnetic Nature:
- If all the molecular orbitals in a molecule are doubly occupied the substance is diagrammatic.
- If one (or) more molecular orbitals are singly occupied it is paramagnetic.
EX: O2 Molecule
Bonding in some Homonuclear diatomic molecules
1. Hydrogen Molecule (H2):
- There are 2 electrons in hydrogen molecule which are present in σ1S molecular orbital so electronic configuration of hydrogen molecule is H2 : (σ1S)2
The bond order of H2 molecule can be calculated as given below
Bond order = [NB - NA] / 2
- Since there is no unpaired electron in hydrogen molecule, therefore it is diamagnetic.
2. Helium molecule (He2):
- In (He2) molecule there are 4 electrons. These electrons will be accommodated in σ1S and σ*1S molecular orbital leading to electronic configuration.
He2 : (σ1S)2 (σ*1S)2
- Bond order He2 is ½ (2-2) = 0
- He2 Molecule is therefore unstable and does not exist.
3. Lithium Molecule (Li2):
- The electronic configuration of lithium is 1S2,2S1. There are six electrons in Li2 the electron configuration of Li2 molecule therefore is Li2 : (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
- There are four electrons present in bonding molecular orbitals and two electrons in anti-bonding molecular orbitals.
Bond order ½ [4 - 1] = 1
- It has no unpaired electrons so it is diamagnetic.
4. Oxygen molecule (O2):
- The electronic configuration of oxygen atom is 1S2 2S2 2P4 each oxygen atom has 8 electrons hence, in O2 molecule there are 16 electrons. The electronic configuration of O2 molecule therefore
O2 : (σ1S) (σ*1S)2 (σ2S)2 (σ*2S)2 (σ2PZ)2 (π2P2x= π2P2y) (π*2P1x = π*2P1y)
- From the electronic configuration of O2 molecule, t is clear that ten electrons are present in anti-bonding molecular orbitals and six electrons are present in anti-bonding molecular orbitals.
Its bond order = ½ [NB - NA] = ½ [10 - 6] = 4
- It may be noted that it contains two unpaired electrons π*2Px and π*2Py molecular orbitals.
- O2 Molecule should be paramagnetic.
5. Nitrogen molecule (N2):
- The electronic configuration of Nitrogen is 1S2 2S2 2P3, there are 14 electrons in N2. The electronic configuration of
N2 : σ1S2 < σ*1S2 < σ2S2 < σ*2S2 < (π2P2x=π2P2y) < σ2P2z
- Bond order of N2 = ½ [10 -4] = 3
- There is no unpaired electron in the electronic configuration of N2
- It is diamagnetic in nature