The mass number of an atom is the total number of protons and neutrons in its nucleus. It is represented by the symbol "A." On the other hand, the atomic mass of an atom is the average mass of all the isotopes of that atom, taking into account their relative abundances. It is represented by the symbol "M" or "m."
The mass of an individual proton and neutron is approximately equal to 1 atomic mass unit (u) or 1 dalton (Da). Since protons and neutrons are the primary constituents of an atom's nucleus, their total mass contributes significantly to the overall atomic mass. Electrons, which are much lighter than protons and neutrons, contribute only a negligible amount to the atomic mass.
Since the mass number (A) represents the sum of protons and neutrons, and each proton and neutron has a mass of approximately 1 atomic mass unit, the mass number is approximately equal to the atomic mass (M) of the atom. This approximation holds true for most atoms, particularly those that do not have a high number of neutrons compared to protons.
However, it's important to note that the atomic mass listed on the periodic table is usually a weighted average of the masses of all the naturally occurring isotopes of an element. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Since different isotopes have different abundances, the atomic mass takes into account their relative proportions. Therefore, in reality, the atomic mass may deviate slightly from the mass number, but the approximation holds true for many elements.