Give reasons for the following :
(a) `CN^(-)` ion is known but `CP^(-)` ion is not known
(b) `NO_(2)` dimerises to form `N_(2)O_(4)`

Explanation

(a) Due to smaller size and higher electronegativity of N than that of P, N can form `p pi - p pi` multiple bonds with C and hence forms `CN^(-)` ion. On the other hand, due to bigger size and lower electronegativity of P than that of N, P does not form multiple bonds with C and hence does not form `CP^(-)` ion.
`underset(("Exists"))(.^(-):C -= N ":") " "underset(("Does not exist"))(.^(-): C - = P ":")`
(b) `NO_(2)` is an odd electron (7 + 8 + 8 = 23 electrons) species with the odd electron present on the N atom. In order to become more stable, the two odd electrons pair up to form a dimer.

(c) The extent of overlapping between orbitals of different halogen atoms is less effective than between orbitals of the same halogen. As a result, I-Cl bond is weaker than Cl-Cl and I-I bonds. In other words, ICl is more reactive than `I_(2)`.