The atomic number of an atom, which represents the number of protons in its nucleus, is always a whole number. It is a fundamental property of an element and defines its identity.
On the other hand, the mass number of an atom represents the total number of protons and neutrons in its nucleus. The mass number can be a whole number or a decimal, depending on the specific isotope of the element. Isotopes are variants of an element that have the same atomic number but different mass numbers due to a varying number of neutrons.
For example, carbon has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. Carbon-12 has a mass number of exactly 12 because it has 6 protons and 6 neutrons. Carbon-13 has a mass number of approximately 13 because it has 6 protons and 7 neutrons. Carbon-14 has a mass number of approximately 14 because it has 6 protons and 8 neutrons.
In summary, while the atomic number is always a whole number, the mass number can be a whole number or a decimal, depending on the specific isotope of an element.