The three principal parts of an atom are

• A. proton, neutron, nucleus
• B. electron, neutron, proton
• C. electron, proton, nucleus
• D. neutron, proton, nucleus

Answer: (B)

Atoms are made of three main subatomic particles: electrons, protons, and neutrons. Protons carry a positive charge, neutrons have no charge, and electrons carry a negative charge. The nucleus at the center holds the protons and neutrons, while the electrons orbit around this nucleus in surrounding space. Most of the atom’s mass comes from the protons and neutrons in the nucleus, while electrons contribute very little mass but determine the atom’s electric behavior and bonding. Because the number of protons equals the number of electrons in a neutral atom, the overall charge is balanced.

So the best choice lists electrons, neutrons, and protons, reflecting the three fundamental subatomic particles that compose ordinary atoms. An option that centers only on the nucleus misses one essential particle (either electrons or a separate particle), and the nucleus itself is not a separate particle but the region where protons and neutrons reside.