Question

Modern atomic mass scale is based on the mass of:
(A) H - 1
(B) C - 12
(C) C - 14
(D) C - 16

Answer 1

Hint: The two isotopes of carbon are C – 12 and C – 13. Each isotope carbon atom has the same number of protons and electrons, 6. But C-12 has 6 neutrons and C-13 has 7 neutrons. So, to define the modern atomic weight scale must specify which C atom represents the scale.

Complete step by step answer:
Generally, atomic mass is defined as the number of protons or electrons of the atom. Since many elements have isotopes, they use average atomic mass. On the periodic table, the mass of carbon was reported as 12.011 amu, which is the average mass of the carbon atom.
If a simple carbon atom is placed in a spectrometer, which will detect C-12 and C-13 atoms. The mass spectrometer gives information about the average mass of the calculated carbon atom.
The mass of C-12 = 12 amu
The mass of C-13 = 13.003355amu, which is 1.0836129 times heavier than C-12.
From the above values, the abundance of C-12 is 99.89%, and the abundance of C-13 is 1.11%
So, the atomic mass on the relative scale is C-12. All the masses of the elements are determined relative to C-12.
Hence, the Modern atomic mass scale is based on the mass of C-12.
So, the correct answer is “Option B”.

Note: The standard unit of the mass of an atom is the atomic mass unit (amu). This is equal to $\dfrac{1}{12}$ the mass of an atom of C-12 equal to $1.6605 x {{10}^{-19}}g$ . The atomic mass unit is also called Avogram, which is a unit of mass and weight equal to one gram divided by Avagadro’s number.