Where is most of an atom's mass located?

• A. In the electron cloud
• B. In the neutrons
• C. In the protons
• D. In the nucleus

Answer: (D)

The mass of an atom is concentrated in the nucleus because the heavy particles there—protons and neutrons—make up nearly all of the atom’s mass, while electrons contribute very little. Protons and neutrons each have roughly the same mass, and together they form the dense center of the atom. The electrons live in the surrounding cloud, but their masses are tiny in comparison—about 1/1836 that of a proton—so they don’t add much to the overall mass. Since the nucleus contains all the protons and neutrons, most of the atom’s mass is located there.