The atomic mass number of an element represents the total number of protons and neutrons in the nucleus of the atom of that element.

It is also called nucleon number or mass number.

Each atom consists of a central nucleus containing protons and neutrons, which is surrounded by a cloud of electrons. Protons are positively charged particles, while neutrons have no charge, so they are neutral.

Electrons, which have a negative charge, orbit around the nucleus at certain energy levels of the seven levels.

The atomic number of an element corresponds to the number of protons in its nucleus.

It identifies the element and locates it in the periodic table.

On the other hand, the atomic mass number is the sum of the number of protons and neutrons in the nucleus.

Because protons and neutrons have similar masses—about one atomic mass unit each—the atomic mass number is often very close to the sum of protons and neutrons.

However, in reality, the atomic mass number is usually a decimal value because it takes into account the presence of different isotopes of an element.

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.
For example, hydrogen has 3 isotopes:

• 99.985% hydrogen-1 (1H), with 0 neutron.
• 0.015% deuterium (2H), with 1 neutrons.
• tritium (3H), with 2 neutrons, radioactive and very rare.

Consequently, the average atomic mass of H1 (1.007825 u)* 99.985 + H2 (2.014102 u) * 0.015 + H3 (3.016049 u) = 1.00797594155 which can be increased to 1.008.

The atomic mass number is crucial for calculating an element's atomic mass, which is the weighted average of the masses of all naturally occurring isotopes.

In short, the atomic mass number represents the total number of protons and neutrons in the nucleus of an atom and plays a vital role in understanding the atomic and isotopic composition of an element.