The concept of hybridization was introduced because the valence bond theory failed to explain the following points :

i. Valencies of certain elements :

The maximum number of covalent bonds which an atom can form equals the number of unpaired electrons present in its valence shell. However, valence bond theory failed to explain how beryllium, boron and carbon forms two, three and four covalent bonds respectively.

a. Beryllium : 

The electronic configuration of beryllium is 1s² 2s². The expected valency is zero (as there is no unpaired electron) but the observed valency is 2 as in BeCl₂.

b. Boron :

The electronic configuration of boron is 1s² 2s² \(2p^1_x\).The valency is expected to be 1 but it is 3 as in BF₃.

c. Carbon : 

The electronic configuration of carbon is 1s² 2s² \(2p^1_x\) \(2p^1_y\) . The valency is expected to be 2, but observed valency is 4 as in CH₄.

ii. The shapes and geometry of certain molecules :

The valence bond theory cannot explain shapes, geometries and bond angles in certain molecules, 

e.g. 

a. Tetrahedral shape of methane molecule. 

b. Bond angles in molecules like NH₃(107°18′) and H₂O (104°35′).

However,

The valency of the above elements and the observe structural properties of the above molecules can be explained by the concept of hybridization. 

These are the reasons for need of the concept of hybridization.