Arrange the following hydrides in decreasing order of their thermal s…

The correct answer is Option (2) → C > A > B > D > E

Down group 16 the size of the central atom increases, so the H–E bond becomes longer and weaker and its bond dissociation enthalpy falls. A weaker bond breaks more easily on heating, so the thermal stability of the hydrides decreases down the group:

    \( \ce{H2O} \) > \( \ce{H2S} \) > \( \ce{H2Se} \) > \( \ce{H2Te} \) > \( \ce{H2Po} \)

Shortcut: the order must begin with \( \ce{H2O} \) (C) and end with \( \ce{H2Po} \) (E). Only Option (2) does both.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Reactivity with Hydrogen – “The increase in acidic character can be explained in terms of decrease in bond (H–E) dissociation enthalpy down the group. Owing to the decrease in bond (H–E) dissociation enthalpy down the group, the thermal stability of hydrides also decreases from H₂O to H₂Po. All the hydrides except water possess reducing property and this character increases from H₂S to H₂Te.”