Classification of Elements and Periodicity in Properties

Which of the following order is wrong?

1. NH3 < PH3 < AsH3 – acidic
2. Li < Be < B < C – 1st IP
3. Al2O3 < MgO < Na2O < K2O – basic
4. Li+ < Na+ < K+ < Cs+ – ionic radius

Answer

(2) Li < Be < B < C – 1st IP

Solution

(1) Acidic character of hydrides increases down group 15 (E–H bond weakens) ⇒ correct.

(2) As per NCERT, B has a lower first ionisation enthalpy than Be (the 2p electron of B is easier to remove than a 2s electron of Be).

Correct order: Li < B < Be < C ⇒ the given order is wrong.

(3) Basic character of oxides increases with metallic character ⇒ correct.

(4) Ionic radius increases down the group ⇒ correct.

Answer: (2)