Rutherford's Proposal About Atom Nuclei

Explanation:

Rutherford's proposal about atoms having positive nuclei was a revolutionary idea in the field of atomic structure. The nucleus of an atom is indeed where most of the mass of an atom is concentrated. It contains positively charged protons and electrically neutral neutrons. The electrons, which are negatively charged, orbit around the nucleus in various energy levels.

The nucleus of the atom contains most of the mass of an atom, as mentioned in option D. Protons and neutrons together make up the majority of the mass of an atom. Electrons, on the other hand, have negligible mass compared to protons and neutrons.

Option B, stating that the nucleus contains protons and electrons, is incorrect as electrons are not found in the nucleus; they are located in the electron cloud surrounding the nucleus. While the size of an atomic nucleus is incredibly small compared to the overall size of an atom, it plays a crucial role in determining the atom's properties like mass and charge.