Q.Discuss the trends in chemical reactivity of group 15 elements.

Step 1: Metallic/nonmetallic character.

Going down the group, metallic character increases: N and P are nonmetals, As and Sb are metalloids, and Bi is essentially a metal. This is reflected in the nature of their oxides — acidic (N, P) → amphoteric (As, Sb) → basic (Bi).

Step 2: Oxidation-state stability.

The relative stability of the $+5$ oxidation state decreases while that of $+3$ increases down the group (inert pair effect), so $Bi_2O_5$/$BiCl_5$ are strong oxidising agents whereas $N_2O_5$/$PCl_5$ are comparatively stable, common compounds.

Step 3: Hydrides ($EH_3$).

Thermal stability of hydrides decreases down the group ($NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$) as the E–H bond weakens with increasing atomic size; correspondingly, their reducing character increases down the group, and their basic strength (ability to donate the lone pair) decreases down the group.

Step 4: Reactivity with oxygen/halogens.

All members combine with oxygen and halogens, but the ease and the type of oxide/halide formed varies systematically with the increasing metallic character and decreasing tendency to form multiple bonds down the group.

Organise the trend by property — metallic character, oxide acid/base nature, hydride stability/basicity/reducing power, oxidation-state stability — and state the direction of change from N to Bi for each.

• A generic "reactivity decreases down the group" statement is too vague — the question wants trend detail across several specific properties (oxides, hydrides, oxidation states).
• Do not mix up which oxidation state becomes more stable — it is $+3$ (not $+5$) that becomes more stable going down the group.
• Do not say hydride basicity increases down the group — it actually decreases (NH₃ is the strongest base of the EH₃ series).