Kaplan Nursing Entrance Exam Practice 2026 – Comprehensive Prep

The majority of an atom's mass is made up of neutrons and protons. Both of these subatomic particles are located in the nucleus of the atom and are significantly more massive than electrons. Each proton has a mass of approximately 1 atomic mass unit (amu), and each neutron has a mass that is also close to 1 amu, whereas an electron has a mass that is about 1/1836 that of a proton. Because electrons orbit the nucleus and contribute very little to the overall mass, the combined mass of the neutrons and protons in the nucleus primarily determines the atom's total mass.

This understanding leads to the conclusion that neutrons and protons are the key contributors to the atomic mass, making that option the correct choice.