Question

Assertion
The average mass of one Mg atom is $24.305amu$, which is not equal to the actual mass of one $Mg$ atom.
Reason
Three isotopes, $24Mg$,$25Mg$ and $26Mg$ of $Mg$ can be found in nature.
A.Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B.Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C.Assertion is correct but Reason is incorrect
D.Assertion is incorrect but Reason is correct

Answer 1

We need to know that the isotopes are variations of a single chemical element that vary in the number of neutrons and, subsequently, in the number of nucleons. Both isotopes of a given element have the same number of protons but different numbers of neutrons in each atom.
Isotopes are variations that differ in the number of neutrons and, therefore, in the number of nucleons of a single chemical element. In each atom, all isotopes of a given element have the same number of protons but different neutron numbers.

Complete step by step answer:
The mean mass of a single atom of Mg is $24.305amu$, which is not the actual mass of a single atom of Mg.
One Mg atom's real mass is either $24amu$ or $25amu$ or$26amu$ .
In nature, three isotopes, $24Mg$ , $25Mg$ and $26Mg$ , are found. They have natural abundances that are different. Their total mass is $24.305amu$.
Thus, both Assertion and Justification are right, and for Assertion, Reason is the correct explanation.
Option A is correct as both the reason and assertion are correct and the reason is the correct explanation.

Note:
We need to remember that the ninth most abundant element in the universe is magnesium. It is formed from the sequential addition of three helium nuclei to a carbon nucleus in large, ageing stars.
Most of the magnesium is ejected into the interstellar medium as those stars erupt as supernovas, where it can be recycled into new star systems. Magnesium is the eighth most common element in the crust of the earth and the fourth most prevalent element in the crust of the earth (after iron, oxygen and silicon) (after iron, oxygen and silicon).