The alpha particles were repelled by electrons. It suggested the nuclear model of the atom. It suggested that atoms are mostly empty space. None of the above is correct. Ordinary chemical reactions do not involve changes in nuclei.
Atomic nuclei are very dense.
Nuclei are positively charged. Electrons contribute only a little to the mass of an atom. The nucleus occupies nearly all the volume of an atom. The atomic number of a certain element is 19, and its atomic weight is 39. Na represents the sum of the number of protons and electrons in the nucleus of a sodium atom.