Ncert Exercises & Solutions

Given below are two statements:

Assertion (A):	One atomic mass unit is defined as one-twelfth of the mass of one carbon-12 atom.
Reason (R):	The carbon-12 isotope is the most abundant isotope of carbon and has been chosen as the standard.

1.	Both (A) and (R) are true and (R) is the correct explanation of (A).
2.	Both (A) and (R) are true but (R) is not the correct explanation of (A).
3.	(A) is true but (R) is false.
4.	(A) is false but (R) is true.

Hint: The carbon atom had been used as a reference while assigning atomic mass to the elements of the periodic table.

According to the standard definition of the atomic unit of mass, it is defined as accurately

\frac{1}{12}

of the mass of a carbon-12 atom. It is true Carbon-12 isotope is the most abundant isotope of carbon and has been chosen as the standard. The percentage of C-12 isotope is 98.93%.

but the correct reason - Carbon-12 is the standard while measuring the atomic masses. Because no other nuclides other than carbon-12 have exactly whole-number masses in this scale.

This is due to two factors:

[1] the different mass of neutrons and protons acting to change the total mass in nuclides with proton/neutron ratios other than the 1:1 ratio of carbon-12; and

[2] an exact whole-number will not be located if there exists a loss/gain of mass to difference in mean binding energy relative to the mean binding energy for carbon-12

Hence, both assertion and reason are true and the reason is not the correct explanation of assertion.

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