Compare the mass of a proton to the mass of an electron.

Electrons are only about 0.054% as massive as neutrons and protons are only 99.86% as massive as the neutrons. The mass of the Proton is 1.67 x 10^-27 kg and the mass of the electron is 9.11 x 10^-31 kg. The mass of the electron is so much lighter than the mass of the proton.

it's c. elements are placed in rows based on how may valencies electrons have.