The atomic number, atomic mass, and proton number are all related to the characteristics of an element but represent different aspects:
Atomic Number: The atomic number of an element is the number of protons found in the nucleus of an atom. Each element has a unique atomic number, which determines its position on the periodic table. For example, hydrogen has an atomic number of 1, indicating it has one proton in its nucleus, while carbon has an atomic number of 6, indicating it has six protons.
Proton Number: The proton number is another term for the atomic number. It represents the number of protons in the nucleus of an atom. So, the atomic number and proton number are essentially the same thing, providing the fundamental identifying characteristic of an element.
Atomic Mass: The atomic mass of an element represents the average mass of the atoms of that element, taking into account the various isotopes and their relative abundances. Since atoms can have different numbers of neutrons, elements can exist in different isotopic forms with the same atomic number but different atomic masses. Atomic mass is typically expressed in atomic mass units (amu) or unified atomic mass units (u). For example, the atomic mass of carbon is approximately 12.01 u because carbon has several isotopes, with carbon-12 being the most abundant.
To summarize, the atomic number and proton number refer to the same concept—the number of protons in an atom's nucleus—while the atomic mass represents the average mass of all the isotopes of that element, taking into account their relative abundances.