Molecular orbital (MO) theory explains the construction of molecular orbital diagram on the basis of following main points .
1.Formation of MOs:
Atomic orbitals(AOs) linearly combine with each other to form equal number of molecular orbitals (MOs).
2.Energy of MOs:
Half of the molecular orbitals (MOs) having energy lower than the atomic orbitals are called as bonding molecular orbitals (BMOs) while half of the molecular orbitals (MOs) having energy higher than the atomic orbitals are called as Anti-bonding molecular orbitals (ABMOs)
3.Total energy remains conserved:
The total energy of the MOs is always equal to the total energy of combining atomic orbitals.
Now let’s understand !!
How to draw molecular Orbital Diagram of Oxygen molecule (O2) ?
Oxygen (O2) molecule:
Oxygen atom has electronic configuration 1s2, 2s2, 2p4 .
Two p-atomic orbitals (one from each oxygen) atom combine to form two molecular orbitals, the bonding molecular orbital σ2px and antibonding molecular orbital σ*2px. The other four p-atomic orbitals (two from each oxygen) atom combine to give four molecular orbitals, two bonding molecular orbitals i.e. π2py and π2pz, while two antibonding molecular orbitals i.e. π*2py and π*2pz. The electron filling in these molecular orbitals follow aufbau, pauli exclusion principle and hund’s rule.
Out of eight electrons six go to bonding molecular orbitals and two to the antibonding molecular orbitals. As electrons are also present in antibonding molecular orbitals so weak bonds will be formed.
Bond order = Number of electrons in BMO – Number of electrons in ABMO
Bond order = 6-2
Bond order = 2
As bond order in Oxygen is 2 so two bonds i.e. double bond is formed between two oxygen atoms (O=O). Further more as there are two unpaired electrons in Oxygen molecule hence it is paramagnetic.
Also Watch Molecular orbital diagram of O2 , O2+2 , 02 –2 ( in Urdu / Hindi)
Simplest Trick to Calculate Bond Order :