$\left.\begin{array}{l} N \\ p \end{array}\right] \rightarrow$ Non metal

$\left.\begin{array}{l} As \\ Sb \end{array}\right] \rightarrow$ Metalloid

$Bi ] \rightarrow$ 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.