Apart from tetrahedral geometry, another possible geometry for CH₄ is square planar with the four H atoms at the corners of the square and the C atom at its centre. Explain why CH₄ is not square planar?

Here consider the electronic configuration of carbon atom:

₆C: 1s² 2s² 2p²

Whereas in the excited stage the orbital diagram can be,

Therefore, the carbon atom undergoes sp³ hybridisation in CH₄ and takes the tetrahedral shape.

But for a square planar shape, the hybridization of the central atom should be dsp² Since the atom of carbon does not have any d-orbitals to undergo hybridisation, the structure of CH₄ cannot be square planar. Also, with a bond angle of 90° in square planar, the stability of CH₄ will be very less because of the repulsion between the bond pairs. Hence, VSEPR theory also favours tetrahedral structure for CH₄.