What is the mass number of an atom?

The mass number of an atom is defined as the sum of protons and neutrons in the nucleus. Protons and neutrons are the two types of nucleons that make up the atomic nucleus, and they contribute significantly to the atom's overall mass.

Protons have a positive charge and contribute to the atomic number, which determines the element's identity. Neutrons, on the other hand, have no charge and also contribute to the mass, but do not affect the element's chemical properties. By adding the number of protons (which represents the atomic number) and the number of neutrons, we get the mass number, which provides a more complete understanding of the atom's structure.

Understanding the mass number is crucial in various areas of physics and chemistry, including nuclear reactions, isotopes, and stability of atoms. The mass number does not include electrons, as they are much lighter and their number does not affect the overall mass of the atom. Hence, the accurate choice relating to the definition of mass number is the sum of protons and neutrons.