Question

The mass of protons and neutrons of an atom make up the atomic mass of the atom, but while counting the atomic mass, why do we add the number of protons and neutrons instead of adding the masses of the same?

Answer 1

The atomic mass of an atom is not simply the sum of the masses of its protons and neutrons. This is because the mass of an atom is determined by the average mass of all its naturally occurring isotopes, taking into account their relative abundances.

Isotopes are different forms of an element that have the same number of protons but different numbers of neutrons. Since isotopes have different masses, the atomic mass of an element is calculated by considering the masses and abundances of all its isotopes.

The atomic mass listed on the periodic table is an average value based on the relative abundance of each isotope of the element. It takes into account the proportion of each isotope present in nature and the mass of each isotope. The atomic mass is weighted by the abundance of each isotope, so isotopes that are more abundant contribute more to the overall average atomic mass.

Therefore, when calculating the atomic mass of an element, we need to consider the mass and relative abundance of each isotope and calculate a weighted average based on those values, rather than simply adding the masses of protons and neutrons.