Q.Explain why NH3 is basic while BiH3 is only feebly basic.

Step 1: Basicity depends on lone-pair availability.

The basic character of $EH_3$ hydrides (E = N, P, As, Sb, Bi) depends on how readily the lone pair on the central atom E can be donated to an acceptor (e.g. $H^+$).

Step 2: Nitrogen.

Nitrogen is small, and its lone pair, held in a compact $sp^3$ hybrid orbital, is close to the nucleus but still spatially accessible and has considerable p-character, making it strongly available for donation. This makes $NH_3$ a good Lewis/Bronsted base (readily accepts $H^+$ to form $NH_4^+$).

Step 3: Bismuth.

Bismuth is a much larger atom. Down the group, due to the inert pair effect, the ns² lone pair increasingly resists participating in bonding/donation — it becomes more tightly held close to the (large, poorly-shielding) nucleus in an orbital with greater s-character, and is spatially far less accessible to an incoming acceptor. This makes the lone pair on Bi a poor donor.

Step 4: Trend across the group. …

Explain that basicity of EH3 hydrides depends on lone-pair availability for donation, then contrast N's small/available lone pair with B …

• Do not attribute the difference simply to electronegativity alone — the inert pair effect (increasing s-character/poor donor ability of the lone pair down the group) is the key reason for Bi specifically.
• BiH₃ is not completely non-basic — it is "feebly" basic, i.e., a very weak base, not a base of zero strength. …