A group 15 element, which is a metal and forms a hydride with...

In group $15$

$] PN \to$ Non-metal

$] SbAs \to$ Metalloid

$Bi ] \to$ Metal

Hydrides of group 15 elements are

${NH}_{3}$

${PH}_{3}$

${AsH}_{3}$

${SbH}_{3}$

${BiH}_{3}$

In ${NH}_{3}$, hydrogen atom gets partial positive charge due to less electronegativity. But in ${BiH}_{3}$, hydrogen atom gets partial negative charge because hydrogen is more electronegative than bismuth.

i.e. ${BiH}_{3}$ is a strong reducing agent than others because we know that ${H}^{-}$ is a strong reducing agent.