Chemical Bonding and Molecular Structure

Molecular Orbital Theory

We know that atoms bond. That results in the diversity of matter around us. But what rules does the atomic or molecular bonding obey? Are there any rules at all? How do you think the molecules are arranged in an element? For that, we need to know the Molecular Orbital Theory. Let us begin!

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Molecular Orbital Theory

The Valence Bond Theory fails to answer certain questions like why He2 molecule does not exist and why O2 is paramagnetic. Therefore in 1932 F. Hood and R.S. Mulliken came up with Molecular Orbital Theory to explain questions like the ones above.

Learn VSEPR Theory to know the geometrical arrangement of various molecules.

According to the Molecular Orbital Theory, individual atoms combine to form molecular orbitals. Thus the electrons of an atom are present in various atomic orbitals and are associated with several nuclei.

Molecular Orbital Theory

We know that we can consider electrons as either particle or wave nature. Therefore, we can describe an electron in an atom as occupying an atomic orbital, or by a wave function Ψ. These are solutions to the Schrodinger wave equation. Electrons in a molecule occupy molecular orbitals. We can obtain the wave function of a molecular orbital by the following methods.

  • Linear Combination of Atomic Orbitals (LCAO)
  • United Atom Method

You can download Molecular Structure Cheat Sheet by clicking on the download button below

hybirdisation cheat sheet

Hybridisation cheat sheet

Linear Combination of Atomic Orbitals (LCAO)

As per this method, the formation of orbitals is because of Linear Combination (addition or subtraction) of atomic orbitals which combine to form the molecule. Consider two atoms A and B which have atomic orbitals described by the wave functions ΨA and ΨB.

If the electron cloud of these two atoms overlaps, then we can obtain the wave function for the molecule by a linear combination of the atomic orbitals ΨA and ΨB. The below equation forms two molecular orbitals.


Bonding Molecular Orbitals

When the addition of wave function takes place, the type of molecular orbitals formed are Bonding Molecular Orbitals. We can represent them by ΨMO = ΨA + ΨB. They have lower energy than atomic orbitals involved.

Anti-Bonding Molecular Orbitals

When molecular orbital forms by the subtraction of wave function, the type of molecular orbitals formed are antibonding Molecular Orbitals. We can represent them as ΨMO = ΨA – ΨB. They have higher energy than atomic orbitals. Therefore, the combination of two atomic orbitals results in the formation of two molecular orbitals. They are the bonding molecular orbital (BMO) and the anti-bonding molecular orbital (ABMO).

Learn Polarity of Molecules and factors on which Polarity depends.

Relative Energies of Molecular Orbitals

  • Bonding Molecular Orbitals (BMO) – Energy of Bonding Molecular Orbitals is less than that of Anti Bonding Molecular Orbitals. This is because of the increase in the attraction of both the nuclei for both the electron (of the combining atom).
  • Anti-Bonding Molecular Orbitals (ABMO) – Energy of Anti Bonding Molecular Orbitals is higher than Bonding Molecular Orbitals. This is because the electron tries to move away from the nuclei and are in a repulsive state.

What happens to an atom and atomic orbital during bonding? Learn Hybridisation to know.

Understand Schrodinger’s Wave Equation

Rules for Filling of Molecular Orbitals

We have to follow certain rules while filling up molecular orbitals with electrons in order to write correct molecular configurations. They are

  • Aufbau Principle – This principle states that those molecular orbitals which have the lowest energy are filled first.
  • Pauli’s Exclusion Principle – According to this principle, each molecular orbital can accommodate a maximum of two electrons having opposite spins.

Read the General Properties of Covalent Compounds here.

Solved Examples for You

Question: Write the Hund’s Rule.

Answer: The Hund’s rule states that in two molecular orbitals of the same energy, the pairing of electrons will occur when each orbital of same energy consist of one electron.

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