Explain with examples $(i)$ Atomic number,$(ii)$ Mass number. Give any two uses of isotopes.

$(a)$ $_{12}^{24} Mg$ and $_{12}^{26} Mg$ are symbols of two isotopes of magnesium. Compare atoms of these isotopes with respect to:
$(i)$ composition of their nuclei.
$(ii)$ electronic configuration and valency.
$(b)$ Give the reason why two isotopes of magnesium have different mass number.

Explain with examples $(i)$ Isotopes and $(ii)$ Isobars. Give any two uses of isotopes.

The number of electrons in an element $X$ is $15$ and the number of neutrons is $16$. Which of the following is the correct representation of the element?

$(a)$ 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.

Isotopes of an element have