Which particles are counted to determine the mass number of an atom?

• A. Protons and neutrons
• B. Protons and electrons
• C. Neutrons and electrons
• D. Only protons

Answer: (A)

Verified step by step guidance

1. Understand that the mass number of an atom is defined as the total number of particles in the nucleus of the atom.
2. Recall that the nucleus contains protons and neutrons, while electrons orbit outside the nucleus and have negligible mass compared to protons and neutrons.
3. Recognize that the mass number is calculated by adding the number of protons and neutrons together, since both contribute significantly to the atom's mass.
4. Note that electrons are not counted in the mass number because their mass is very small and does not affect the overall mass significantly.
5. Therefore, the particles counted to determine the mass number are protons and neutrons.