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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? 

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Electronic configuration of carbon atom: 

6C:1s2 2s2 2p2

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

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 . However, an atom of carbon does not have d-orbitalsto 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

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