An atom's atomic mass is best described as the mass of:

• A. protons and neutrons in the atom
• B. only the electrons in the atom
• C. only the neutrons in the atom
• D. all protons, neutrons, and electrons in the atom

Answer: (D)

Verified step by step guidance

1. Understand that the atomic mass of an atom is primarily determined by the particles that contribute most to its mass: protons, neutrons, and electrons.
2. Recall that protons and neutrons each have a mass of approximately 1 atomic mass unit (amu), while electrons have a much smaller mass (about 1/1836 of a proton).
3. Recognize that because electrons have such a small mass compared to protons and neutrons, their contribution to the total atomic mass is negligible.
4. Therefore, the atomic mass is best described as the combined mass of protons and neutrons in the atom, not including electrons in any significant way.
5. Conclude that the correct description of atomic mass is the mass of protons and neutrons in the atom.