In nitrogen family the H-M-H bond angle in the hybrides MH3 gradually becomes closer to 90o on going from N to Sb. This shows that gradually
(A) The basic strength of hybrides increases
(B) Almost pure p-orbitals are used for M-H bonding
(C) The bond energies of M-H bond increases
(D) The bond pairs of electrons becomes nearer to the central atom
The hybrides have a pyramidal or tetrahedral shape with a lone pair of electrons in on of the orbitals.The H-M-H bond angle is less than the original 109o28' tetrahedral bond angle(H-N-H in NH3 is 106o45') because of greater repulsion between lone pair and a bond pair than between two obnd pairs of electrons.Because electronegativity of M decreases from N to Bi, the bond pair lie farther away from the central atom, and the lone pair causes greater distortion of bond angle.
Thus H-P-H bond in PH3 is 94o , while in AsH3 and SbH3 it is about 91.8o and 91.3o respectively(closer to 90o). This suggests that orbitals used for bonding are closer to pure p-orbitals.