No, the atomic mass number is not always a whole number. The atomic mass number, also known as the mass number, is the sum of the number of protons and neutrons in the nucleus of an atom. While protons and neutrons both have masses close to one atomic mass unit (u), they are not exactly equal. Neutrons have a slightly greater mass than protons.
Since the atomic mass number is the sum of these two values, it is typically a decimal number. However, on the periodic table of elements, the atomic mass listed for each element is often a weighted average of the masses of all the naturally occurring isotopes of that element. These isotopes have different numbers of neutrons, resulting in decimal values for the atomic mass.