In atomic theory, what do the atomic number and mass number tell you about an element?

• A. The atomic number equals the total number of nucleons; the mass number equals the number of protons.
• B. The atomic number Z equals the number of protons and defines the element; the mass number A equals protons plus neutrons.
• C. The atomic number equals the number of electrons; the mass number equals the number of neutrons.
• D. The atomic number equals the total number of electrons plus protons.

Answer: (B)

The main idea is that two numbers describe different aspects of an element’s nucleus and identity. The atomic number tells how many protons are in the nucleus, and that count uniquely defines the element. The mass number tells how many nucleons there are in total, so it’s the sum of protons and neutrons. This is why elements can have isotopes: the same number of protons (same element) but different numbers of neutrons (different mass numbers).

In a neutral atom, the number of electrons equals the number of protons, but the mass number does not involve electrons.