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

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

Answer: (A)

Verified step by step guidance

1. Step 1: Understand that the atomic number of an atom is defined as the number of protons present in its nucleus. This number uniquely identifies an element.
2. Step 2: Recognize that the mass number of an atom is the total count of protons and neutrons in the nucleus, which together contribute most of the atom's mass.
3. Step 3: Note that electrons have negligible mass compared to protons and neutrons, so they are not included in the mass number.
4. Step 4: Differentiate between atomic number and mass number by remembering that atomic number = number of protons, while mass number = number of protons + number of neutrons.
5. Step 5: Use this understanding to evaluate the given options and identify the correct description of the difference between atomic number and mass number.