a) ²⁴Mg₁₂ and ²⁶Mg₁₂ are symbols of two isotopes of magnesium. Compare the atoms of these isotopes with respect to :

(1) the composition of their nuclei.

(2) their electronic configurations.

(3) give the reason why the two isotopes of magnesium have different mass

numbers.

Atomic number or proton number=Z, Mass Number =A, neutron number (n)=A-Z
​for ²⁴Mg₁₂  :
 A=24
Z=12
n=24-12=12

​for ²⁶Mg₁₂  :​
A=26
Z=12
n=26-12=14

2) electronic configuration
1s² 2s² 2p⁶ 3s²

Given picture gives the some information

1) 24Mg12 = protons = 12, Neutrons = 12 26Mg12 = prtons = 12, Neutrons = 14 2) Both have Same Electronic Configurations = 2,8,2 3)Isotopes are variations in the element that have different numbers of (neutral) neutrons. Because neutrons have no effect on charge, they have no effect on the electronic and bonding properties of the element, but affect the atomic weight.