Question

Assertion: Actual atomic mass is greater than the mass number.
Reason: Mass of protons and neutrons is measured in daltons or atomic mass unit (u) but the mass of each neutron and proton is slightly greater than I daltons.
A.Both Assertion and Reason are true and Reason is the correct explanation to Assertion.
B.Both Assertion and Reason are true and Reason is not the correct explanation to Assertion.
C.The assertion is true but the Reason is false.
D.Both assertion and reason are incorrect.

Answer 1

The atomic mass of an element is defined as the average relative mass of its atom as compared with an atom of carbon taken as 12 or we can say the average atomic mass of an element is the number of times an atom of that element is heavier than an atom of carbon taken as 12.

Complete step by step answer:
As we know the mass number of an atom is equal to the sum of the number of protons and the number of neutrons and the mass of each neutron and proton is measured in Dalton or atomic mass unit. But in actual practice, the mass of each proton and neutron is slightly greater than one Dalton. Hence we can say the actual mass is slightly greater than the mass number. So from the above explanation, we can say both assertion and the reason are true and the reason is the correct explanation of assertion.
So the correct answer is option A.

Additional information: The mass of an atom is expressed in terms of atomic mass unit and one atomic mass unit is equal to $\dfrac{1}{{12}}th$ the mass of an atom of carbon-12 isotopes. During recent years, the atomic mass of an element has been measured accurately with the help of an instrument which is known as a mass spectrometer.

Note:
Alternatively, the average relative mass of an element is calculated from the fractional abundances of the isotopes of that element. Here the fractional abundances of isotopes are defined as the fraction of the total number of atoms that is composed of that particular isotope.