What is true about the mass number of an atom?

The mass number of an atom is defined as the total number of protons and neutrons located in the nucleus of that atom. This number provides important information about the atom's mass and nuclear composition. Protons and neutrons collectively account for nearly all of an atom's mass, while electrons, which exist in the surrounding electron cloud, contribute very little to the mass due to their significantly lower mass in comparison.

Understanding that the mass number is solely a function of the counts of protons and neutrons highlights its intrinsic relationship to atomic structure. As isotopes of an element can have the same number of protons but differing numbers of neutrons, they will have different mass numbers while maintaining the same elemental identity. This distinction is crucial in the study of chemistry and physics, especially regarding nuclear reactions and stability.

In summary, the mass number provides a fundamental piece of information about an atom that reflects its composition, specifically the quantity of protons and neutrons, thus making this a key concept in understanding atomic structure and behavior.