Question

Give the reason why the two isotopes of magnesium have different mass numbers?

Answer 1

Isotopes are atoms that share the same atomic number but have distinct mass numbers. The total of the number of neutrons and protons present inside the nucleus is the mass number. The mass of an element is determined by the number of neutrons and protons.

Complete answer:
The atomic number refers to the number of protons or electrons in an atom. Isotopes are several forms of the same atom that change only in their number of neutrons. Each element's atoms contain a specific number of protons, however the number of neutrons for a given element can vary.
${}_{12}^{24}Mg$ and ${}_{12}^{26}Mg$ are two isotopes of $Mg$ .
Since isotopes differ by the number of neutrons, the mass number of isotopes of $Mg$ are different.
In ${}_{12}^{24}Mg$ ,
Number of protons $= 12$
Number of neutrons $= 12$
In ${}_{12}^{26}Mg$ ,
Number of protons $= 12$
Number of neutrons $= 14$ .

Additional Information:
Isotope analysis is used to determine the isotopic signature of element samples, which is an essential use of isotopes. Isotope ratio mass spectrometry is commonly used to do this.
Isotopic substitution can be used to determine the mechanism of a chemical process. The kinetic isotope effect can be used to assess the change in reaction rate.
Isotope dilution can also be used to determine the concentration of a variety of elements/substances.

Note:
Isotopes are atoms with the same number of protons but varying numbers of neutrons, whereas isobars are atoms with the same mass number. Isotones are atoms that have the same number of neutrons but differ in the number of protons they have.