8. The sum of the number of protons and neutrons in an atom is called the _______________.

The sum of the number of protons and neutrons in an atom is called the mass number.

The mass number is a fundamental property of an atom that represents the total count of nucleons in the atomic nucleus. Protons are positively charged particles, while neutrons are neutral particles. The mass number is crucial because it directly influences the atom's stability and its isotopes.

For instance, an element like carbon has 6 protons. If it has 6 neutrons, its mass number is 12 (6 protons + 6 neutrons). However, carbon can also exist as isotopes, such as carbon-14, which has 6 protons and 8 neutrons, resulting in a mass number of 14. Understanding the mass number is essential for studying atomic structure, nuclear reactions, and various applications in fields like chemistry, physics, and medicine.