in an atom, most of the mass is in the __________. what one word completes the sentence?

The one word that completes the sentence "In an atom, most of the mass is in the ________." is "nucleus." Most of an atom's mass is concentrated in the nucleus because it contains protons and neutrons, which are much heavier than electrons. The electrons occupy almost all of the atom's volume but contribute very little to its mass. The nucleus is extremely small compared to the overall size of the atom but holds the vast majority of its mass.