$(a)$ $_{12}^{24} Mg$ 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.

(N/A) $(a) (i)$ For $_{12}^{24} Mg$: Protons $(P) = 12$,Neutrons $(N) = 24 - 12 = 12$.
For $_{12}^{26} Mg$: Protons $(P) = 12$,Neutrons $(N) = 26 - 12 = 14$.
$(ii)$ Both isotopes have the same atomic number $(12)$,so their electronic configuration is $2, 8, 2$ and their valency is $2$.
$(b)$ Isotopes have different mass numbers because they possess a different number of neutrons in their nuclei,even though the number of protons remains the same.