Which of the following best describes the difference between atomic number and mass number?

• A. Atomic number is the number of neutrons, while mass number is the number of protons.
• B. Atomic number is the total number of particles in the nucleus, while mass number is the number of electrons.
• C. Atomic number is the sum of protons and electrons, while mass number is the number of neutrons.
• D. Atomic number is the number of protons in an atom, while mass number is the total number of protons and neutrons.

Answer: (D)

Verified step by step guidance

1. Understand that the atomic number (Z) of an atom is defined as the number of protons in its nucleus. This number uniquely identifies an element.
2. Recognize that the mass number (A) is the total count of protons and neutrons in the nucleus of an atom, representing the atom's mass in atomic mass units (approximately).
3. Note that electrons are not included in the mass number because their mass is negligible compared to protons and neutrons.
4. Compare the given options by checking which correctly states that atomic number equals the number of protons, and mass number equals the sum of protons and neutrons.
5. Conclude that the correct description is: Atomic number is the number of protons in an atom, while mass number is the total number of protons and neutrons.