What is meant by hybridisation ? Compound `CH_(2)=C=CH_(2)` contains `sp` or `sp^(2)` hybridised carbon atoms. Will it be a planar molecule ?

Explanation

Hybridisation is mixing of atomic orbitals to form new hybrid orbitals. The new orbital have the same total electron capacity as the old ones. The properties and energies of the new hybridised orbitals are an average of the unhybridised orbitals.
`{:(overset(1)(C)H_(2) = overset(2)(C) = = overset(3)(C)H_(2)),(" " 3 sigma " " 2 sigma " " 3 sigma ):}`
Hybridisation can be found by counting numbeer of `sigma`- bonds around the carbon atom.
`{:(3 sigma = sp^(2) " hybridisation "),(2 sigma = sp " hybridisation "):}`
In allene, carbon atoms 1 and 3 are `sp^(2)`-hybridised as each one of them is joined by a double bond. And, carbon atom 2 is sp-hybridised as it has two double bonds at each of its side. Therefore, the two `pi-`bonds are perpendicular to each other, in allene, as shown below.

`H_(a)` and `H_(b)` lie in the plane of paper while `H_(c) " and " H_(d)` lie in a plane perpendicular to the plane of the paper. Hence, the allene molecule as a whole is non-planar.