$(a)$ What are isobars?
$(b)$ Atomic number of an element $Y$ is $17$.
$(i)$ Write its electronic configuration.
$(ii)$ What is the number of valence electrons in $Y$?
$(iii)$ How many electrons are needed to complete the octet of $Y$?
$(iv)$ Is it a metal or non-metal?
$(c)$ The valency of $Na$ is $1$ and not $7$. Give reason.

(D) Atoms of different elements with different atomic numbers but the same mass number are called isobars.
$(b)$ $(i)$ The electronic configuration of $Y$ $(Z=17)$ is $2, 8, 7$.
$(ii)$ The number of valence electrons in $Y$ is $7$.
$(iii)$ To complete the octet,$Y$ needs $8 - 7 = 1$ electron.
$(iv)$ Since $Y$ has $7$ valence electrons,it tends to gain $1$ electron to complete its octet,making it a non-metal.
$(c)$ The electronic configuration of $Na$ $(Z=11)$ is $2, 8, 1$. To achieve a stable noble gas configuration,it is energetically easier for $Na$ to lose its $1$ valence electron than to gain $7$ electrons. Therefore,its valency is $1$ and not $7$.