What does the mass number represent in an atom?

The mass number of an atom is defined as the sum of protons and neutrons in the nucleus. Protons are positively charged particles, while neutrons have no charge, and together they contribute to the overall mass of the atom. Since electrons have a negligible mass compared to protons and neutrons, they do not factor into the mass number.

When considering the context of atomic structure, the mass number provides important information about the identity and stability of the atom. For instance, isotopes of an element have the same number of protons but different mass numbers due to differing numbers of neutrons. Understanding the mass number is crucial for various applications, such as in nuclear chemistry, where the stability and behavior of isotopes are significant.

The other options do not accurately describe the mass number: the number of electrons (which defines the atom's charge and not its mass), the atomic weight (which is averaged based on isotopic distribution and not a straightforward sum of particles), and the total volume of an atom (which is related to electron distribution and not directly to mass).