$^{24}${12}\text{Mg} and $^{26}{12}\text{Mg}$ are symbols of two isotopes of magnesium. Compare the atoms of these isotopes with respect to :

(i) the composition of their nuclei.

(ii) their electronic configurations.

(iii) give the reason why the two isotopes of magnesium have different mass numbers.

(i) The composition of their nuclei :

$^{24}_{12}\text{Mg}$ :

Atomic number [Z] = electron [e] = proton [p] = 12

Mass number [A] = 24

As, neutron [n] = A - Z = 24 - 12 = 12.

Hence, p = 12, n = 12

$^{26}_{12}\text{Mg}$ :

Atomic number [Z] = electron [e] = proton [p] = 12

Mass number [A] = 26

As, neutron [n] = A - Z = 26 - 12 = 14.

Hence, p = 12, n = 14

(ii) Their electronic configurations :

$^{24}_{12}\text{Mg}$ :

As electron [e] = 12 hence, electronic configuration = 2,8,2

$^{26}_{12}\text{Mg}$ :

As electron [e] = 12 hence, electronic configuration = 2,8,2

(iii) Reason for difference in mass numbers — As the number of neutrons is different in the two isotopes of magnesium, therefore, they have different mass numbers.