Question

Assertion:Magnesium has two isotopes $_{12}^{24}Mg$and $_{12}^{26}Mg$. According to which isotopes of magnesium have different mass numbers.
Reason: Different mass number of isotopes is due to the difference in neutrons no.
(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) Both Assertion and Reason are incorrect.

Answer 1

Hint: Those atoms which have the same atomic number but different mass numbers are known as isotopes. The mass number gives us the sum of the number of neutrons and protons present inside the nucleus.

Complete answer:
 - As we know, the number of protons or electrons in an atom is called the atomic number. The atoms of each element contain a characteristic number of protons while the number of neutrons for a given element can vary and the forms of the same atom that differ only in their number of neutrons are known as isotopes.
- The number of neutrons and the number of protons determine the element’s mass number (i.e. mass number = protons + neutrons). If we are asked to calculate how many neutrons an atom has, we can just subtract the number of protons, or atomic number, from the mass number.
- In $_{12}^{24}Mg$, 12 is the atomic number (no. of protons) and 24 is the mass number. Since mass number (24) is the sum of the number of protons (12) and neutrons, the number of neutrons will be 24−12=12. Therefore the number of neutrons in $_{12}^{24}Mg$ is 12.
- In $_{12}^{26}Mg$, 12 is the atomic number (no. of protons) and 26 is the mass number. The number of neutrons will be 26−12=14. Therefore the number of neutrons in $_{12}^{26}Mg$ is 14.
Since $_{12}^{26}Mg$ and $_{12}^{24}Mg$ have different numbers of neutrons, they will have different mass numbers. Therefore Reason is the correct explanation for Assertion and both reason and assertion are correct.

Thus the answer is option (A).

Note: Do not confuse between the terms isotopes, isobars and isotones. Atoms which have the same number of protons, but different numbers of neutrons are isotopes while Isobars are atoms having the same mass number are isobars. Isotones are those atoms which have the same number of neutrons but a different number of protons.