The mass of a proton is closest to the mass of which subatomic particle?

• A. Neutron
• B. Alpha particle
• C. Positron
• D. Electron

Answer: (A)

Verified step by step guidance

1. Recall the three main subatomic particles: protons, neutrons, and electrons, and also consider the positron, which is the electron's antiparticle.
2. Understand that the mass of a proton is approximately 1 atomic mass unit (amu), which is roughly 1.67 \times 10^{-27} kilograms.
3. Compare the masses of the given particles: a neutron has a mass very close to that of a proton, while electrons and positrons have masses about 1/1836 that of a proton, making them much lighter.
4. An alpha particle consists of 2 protons and 2 neutrons, so its mass is roughly 4 times that of a proton, which is significantly heavier.
5. Conclude that since the neutron's mass is closest to the proton's mass, the correct answer is neutron.