The atomic mass number, also known as the mass number, represents the total number of protons and neutrons in an atom's nucleus. It is a whole number because protons and neutrons have defined masses that are approximately whole numbers when expressed in atomic mass units (amu).
Protons and neutrons are subatomic particles that contribute to the mass of an atom. Each proton has a mass of approximately 1 amu, and each neutron also has a mass of approximately 1 amu. Since the atomic mass number is simply the sum of the number of protons and neutrons, it will be a whole number because we are adding whole numbers together.
For example, if an atom has 6 protons and 6 neutrons, the atomic mass number would be 12. This means that the atom has an atomic mass of approximately 12 atomic mass units.
It's important to note that the actual masses of protons and neutrons are slightly less than 1 amu, but for practical purposes, they are often approximated to 1 amu. In reality, the atomic masses listed on the periodic table are based on more precise measurements that take into account the actual masses of isotopes and their abundances.