Question

One atomic unit (a.m.u.) is equal to:
(A) 1 eV of energy
(B) 931 eV of energy
(C) 1 MeV of energy
(D) 931 MeV of energy

Answer 1

Hint : An atomic mass unit is defined as a mass equal to one-twelfth the mass of an atom of carbon- $12$ . The mass of an isotope of any element is expressed in relation to the carbon- $12$ standard. For example, one atom of helium- $4$ has a mass of $4.0026$ amu.

Complete Step By Step Answer:
Masses of individual atoms are very, very small. Using a modern device called a mass spectrometer, it is possible to measure such minuscule masses. An atom of oxygen- $16$ , for example, has a mass of $2.66 \times {10^{ - 23}}g$ . While comparisons of masses measured in grams would have some usefulness, it is far more practical to have a system that will allow us to more easily compare relative atomic masses. Scientists decided on using the carbon- $12$ nuclide as the reference standard by which all other masses would be compared. By definition, one atom of carbon- $12$ is assigned a mass of $12$ atomic mass units (amu).
E = m{C^2} \\\ m = 1.66 \times {10^{ - 27}}Kg \\\ C = 3.8 \times {10^8}m/s \\\ E = 1.66 \times {10^{ - 27}} \times {(3.8 \times {10^8})^2} \\\ E = 1.49 \times {10^{ - 10}}J \\\ 1MeV = 1.6 \times {10^{ - 13}}J \\\ E = 931.25 \approx 931MeV \\\
So, option D is correct.

Note :
The carbon- $12$ atom has six protons and six neutrons in its nucleus for a mass number of $12$ . Since the nucleus accounts for nearly all of the mass of the atom, a single proton or single neutron has a mass of approximately 1 amu. However, as seen by the helium and sulfur examples, the masses of individual atoms are not whole numbers. This is because an atom’s mass is affected very slightly by the interactions of the various particles within the nucleus, and the small mass of the electron is taken into account.