What is the correct definition of mass number?

The definition of mass number is the sum of protons and neutrons in an atom's nucleus. This total provides crucial information about the nucleus's composition and is essential for understanding the atom's overall mass, as protons and neutrons contribute significantly to the mass, whereas electrons have negligible mass compared to them.

The mass number distinguishes between isotopes of an element, which have the same number of protons but different numbers of neutrons. For example, carbon-12 has six protons and six neutrons, resulting in a mass number of 12. This concept is foundational in chemistry and nuclear physics, as it helps in categorizing elements and understanding their behaviors during chemical reactions.