Question

Suppose that methane (CH4) is square planar instead of tetrahedral. Determine the characters of the H 1s orbitals and use the projection operator method to determine the SALCs. Draw an interaction diagram. Explain why methane is not square planar.

Answer 1

The phenomenon of mixing of orbitals of the same atom with slight difference in energies so as to redisitribute their energies & give new orbitals of equivalent energy & shape. The new orbitals formed are known as hybrid orbitals & is called as hybridization.

Now, Electronic configuration of carbon atom:

C: 1s² 2s² 2p²

In the excited state, the orbital picture of carbon can be represented as:

Here 1 s orbital & 3 p orbitals under goes hybridization to form sp³ hybrid orbitals .Hence, carbon atom undergoes sp³ hybridization in CH₄ molecule and takes a tetrahedral shape.

For a square planar shape, the hybridization of the central atom has to be dsp², ie, 1 s orbital, 3 p orbitals & 1 d orbitals has to undergo hybridization .However, an atom of carbon does not have d-orbitals to undergo dsp² hybridization. Hence, the structure of CH₄ cannot be square planar.

Moreover, with a bond angle of 90° in square planar, the stability of CH₄ will be very less because of the repulsion existing between the bond pairs. Hence, VSEPR theory also supports a tetrahedral structure for CH₄.