CBSE Class 12 Chemistry P Block Elements Notes Set A

1.Account for the following: (Group 15 elements)

(i) There is a considerable increase in covalent radius from N to P. However, from As to Bi only small increase in covalent radius is observed.

Ans: This is due to the presence of completely filled d and/or f orbital in heavier members which have low screening effect.

(ii) Ionization enthalpy decreases down the group 15.

Ans: Due to gradual increase in atomic size.

(iii) The ionization enthalpy of the group 15 elements is much greater than that of group 14 elements in the corresponding periods.

Ans: Because of the extra stable half-filled p orbital electronic configuration and smaller size.

(iv) Nitrogen exists as gas and phosphorus as solid.

Ans: Because N₂ is Diatomic molecules, hence weak Vander Waal’s force of attraction thus is a gas whereas P₄ is tetra atomic hence Stronger Vander Waal’s force of attraction thus it is solid.
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(v) NH3 is basic while BiH3 is only feebly basic.

Ans: NH₃ is basic due to smaller size & high electron density on lone pair of Nitrogen.

(vi) The HNH angle value is higher than HPH, HAsH, and HSbH angles.

Ans: Due to increase in repulsion by the lone pair with increase in size of lone pair from N to Sb.

(vii) R₃P=O exist but R3N=O does not.

Ans: Due to the absence of d orbitals in valence shell of nitrogen. Because of inability of Nitrogen to expand its covalency beyond four, nitrogen cannot form d π–p π bond.

(viii) Nitrogen shows catenation properties less than phosphorus.

Ans: Because N-N bond is weaker than the single P-P bond.& strong pπ–pπ overlap in Nitrogen.

(ix) Ammonia has higher boiling point than Phosphine.

Ans: Ammonia (NH3) form hydrogen bond but Phosphine (PH₃) does not.

(x) H3PO3 is diprotic acid.

Ans: Due to presence of two ionisable –OH group. (Draw the structure of H₃PO₃).

(xi) Oxides of nitrogen have open chain structures while those of phosphorus have closed chain or cage structures

Ans:Nitrogen has ability to form multiple bonds involving p-p overlap while Phosphorous does not.

(xii) A nitrogen atom has five valence electrons but it does not form the compound NCl5.

Ans: Because of absence of d-orbitals it can’t expand its covalency from 3 to 5.. 

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