The atomic mass number is the total number of protons and neutrons in an atom.

• A. Atomic Number
• B. Atomic Mass Number
• C. Buffer
• D. Solvent

Answer: (B)

The mass number represents the total count of nucleons in the nucleus, which are the protons and neutrons. This sum determines the isotope of an element, since electrons contribute negligible mass and are not included in the mass number. The atomic number, by contrast, counts only protons and defines the element’s identity and usual charge. The other options don’t describe this concept: a buffer is a solution that resists pH change, and a solvent is the liquid in which substances dissolve. For example, carbon-12 has 6 protons and 6 neutrons (mass number 12), while carbon-14 has 6 protons and 8 neutrons (mass number 14).