The mass number is always a whole number because it represents the total count of protons and neutrons in an atom's nucleus, and both protons and neutrons are discrete particles that can only exist in whole numbers. Since the mass number is the sum of these two counts, it must also be a whole number
. To elaborate:
- The mass number (symbol AAA) is defined as the total number of protons plus neutrons (collectively called nucleons) in the nucleus of an atom
- Protons and neutrons are individual subatomic particles; you cannot have a fraction of a proton or neutron in a nucleus, so their numbers are always integers
- Therefore, when you add the number of protons and neutrons, the resulting mass number is always an integer (whole number)
This differs from atomic mass (or relative atomic mass), which is often not a whole number because it is a weighted average of the masses of all naturally occurring isotopes of an element, each with different numbers of neutrons and slightly different masses due to nuclear binding energy effects
. In summary, the mass number is always a whole number because it counts actual particles (protons and neutrons), which cannot be fractional.
