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

• A. Atomic number is the number of neutrons, while atomic mass is the number of electrons.
• B. Atomic number is the average mass of all isotopes, while atomic mass is the number of protons.
• C. Atomic number is the sum of protons and electrons, while atomic mass is the number of neutrons.
• D. Atomic number is the number of protons in an atom, while atomic mass is the total number of protons and neutrons.

Answer: (D)

Verified step by step guidance

1. Understand that the atomic number of an element is defined as the number of protons in the nucleus of an atom. This number uniquely identifies the element.
2. Recognize that the atomic mass (or mass number) is the total count of protons and neutrons in the nucleus of an atom, which together contribute most of the atom's mass.
3. Note that electrons have negligible mass compared to protons and neutrons, so they do not significantly affect the atomic mass.
4. Recall that isotopes of an element have the same atomic number (same number of protons) but different atomic masses due to varying numbers of neutrons.
5. Therefore, the correct distinction is: atomic number = number of protons, and atomic mass = number of protons + number of neutrons.