Why is mass number a decimal?
Mass number is not a decimal number, it is a whole number. Mass number refers to the number of protons and neutrons in the nucleus of an isotope of an element, and is a whole number.
For example, carbon-14 is an isotope of carbon. Its mass number is 14. This means that the sum of the protons and neutrons in the nucleus is 14. Since carbon's atomic number is 6, the number of protons is 6. The mass number 14 minus the 6 protons equals 8 neutrons.