What is defined as the total number of protons plus neutrons in an atom?

The total number of protons plus neutrons in an atom is known as the mass number. This value represents the combined count of these two subatomic particles, which are located in the nucleus of the atom. Protons carry a positive charge and define the element’s identity through the atomic number, while neutrons are neutral and contribute to the mass of the atom but not its chemical identity.

Understanding the mass number is crucial because it helps distinguish between different isotopes of an element, which have the same number of protons but varying numbers of neutrons. This concept is key in various fields, such as nuclear chemistry, where isotopic variations can influence stability and reactivity. Other terms in the choices, such as atomic number and atomic mass, refer to different concepts related to atomic structure and properties, which do not represent the sum of protons and neutrons.