What does the atomic mass tell us about an atom?

The atomic mass of an atom provides the total number of protons and neutrons present in its nucleus. This value gives insight into the overall mass of the atom since protons and neutrons contribute significantly to the atomic mass, while electrons contribute minimally due to their much smaller mass.

Understanding atomic mass is critical because it helps in determining the isotopes of an element, which have the same number of protons but a different number of neutrons. By knowing the atomic mass, one can identify the different isotopes of an element and their relative abundance. This is particularly important in fields such as chemistry and nuclear physics, where isotopic composition can impact reactions and stability.

In contrast to other aspects mentioned in the options, atomic mass does not reflect the number of electrons alone, as the number of electrons in a neutral atom is equal to the number of protons. Similarly, atomic mass does not indicate an atom's reactivity directly, as reactivity is more influenced by the electron configuration rather than the total mass of protons and neutrons. Thus, the correct understanding of atomic mass is foundational in characterizing the elements and their behavior in chemical reactions.