Where is most of the mass of an atom located?

The mass of an atom is predominantly located in the nucleus, which contains protons and neutrons. Protons are positively charged particles, while neutrons have no charge. Both of these particles have a much greater mass compared to electrons, which are found in the electron cloud surrounding the nucleus.

The nucleus is extremely small in size relative to the overall atom, yet it holds the vast majority of the atom's mass—approximately 99.9%. This is because the contributions of the electrons to an atom's mass are negligible when compared to those of protons and neutrons.

The other choices focus on components of the atom that contribute little to its mass or describe aspects of atomic structure that do not hold mass themselves. For instance, the electron cloud encompasses the area where electrons reside but contains very little mass due to the light nature of electrons. Orbital shells refer to the energy levels where electrons exist, but they do not inherently contain mass themselves. Photons, being massless particles of light, do not contribute to the mass of atoms at all. Hence, the nucleus stands out as the correct answer when considering where the majority of an atom's mass is located.