Which subatomic particles contribute to an atom's mass number but not its atomic number?

• A. Electrons
• B. Quarks
• C. Protons
• D. Neutrons

Answer: (D)

Verified step by step guidance

1. Recall that the atomic number of an atom is defined as the number of protons in its nucleus. This number determines the element's identity.
2. Understand that the mass number of an atom is the total count of protons and neutrons in the nucleus, representing the atom's overall mass.
3. Recognize that electrons have negligible mass compared to protons and neutrons, so they do not significantly contribute to the mass number.
4. Note that protons contribute to both the atomic number and the mass number because they are counted in both definitions.
5. Conclude that neutrons contribute to the mass number by adding to the nucleus's mass but do not affect the atomic number since they have no charge.