Elements $X, Y$ and $Z$ have $4, 5$ and $7$ valence electrons respectively.
$(a)$ Write the molecular formula of the compounds formed by these elements individually with hydrogen.
$(b)$ Which of these compounds will have the highest dipole moment?

(B) The valency of an element is determined by its valence electrons. For $X$ (valence electrons = $4$),the valency is $4$,so it forms $XH_4$. For $Y$ (valence electrons = $5$),the valency is $3$,so it forms $YH_3$. For $Z$ (valence electrons = $7$),the valency is $1$,so it forms $ZH$.
$(b)$ Dipole moment depends on the electronegativity difference between the bonded atoms. $Z$ is a halogen (group $17$) and is the most electronegative among the three. Therefore,the bond $H-Z$ has the highest polarity,resulting in the highest dipole moment for $ZH$.