Which orbital is non directional?
They are non directional because they are spherical in shape and probability of finding electrons at a particular distance from the nucleus is same in all directions.
Define hybridization and explain why was it introduced?
Hybridization may be defined as the phenomenon of combining of orbitals of an atom of nearly equal energies, giving rise to entirely new orbitals equal in number to the mixing orbitals having same energy and identical shapes.
By valence bond theory, only half filled orbitals can take part in bonding.
Hybridization was introduced to explain molecular structure when the valence bond theory failed to correctly predict them.
Taking example of a CH4 molecule,
without hybridization concept, the orbitals of the outermost shell of carbon would be as shown, with the p orbitals 90o with eachother when in fact, the C--H bonds are each 109.28o .
And hence, by formation of 4 identical hybrid orbitals, the geometry of CH4 molecule can be explained.
However, it should be kept in mind that this is a hypothetical concept.