Which statement correctly describes the difference between mass number and atomic number for an atom?

• A. Mass number and atomic number both represent the total number of particles in the atom.
• B. Mass number is the total number of protons and neutrons in the nucleus, while atomic number is the number of protons only.
• C. Mass number is the number of electrons, while atomic number is the number of neutrons.
• D. Mass number is always smaller than atomic number for all elements.

Answer: (B)

Verified step by step guidance

1. Understand that the atomic number of an atom is defined as the number of protons in its nucleus. This number uniquely identifies the element.
2. Recognize that the mass number is the total count of protons and neutrons in the nucleus of the atom, representing the atom's total nuclear mass in terms of nucleons.
3. Note that electrons are not included in either the atomic number or the mass number because their mass is negligible compared to protons and neutrons.
4. Compare the two: atomic number = number of protons, mass number = number of protons + number of neutrons.
5. Conclude that the correct description is: mass number is the total number of protons and neutrons in the nucleus, while atomic number is the number of protons only.