Today in class, I learned about Atom. What is atom? There are 3 states of atoms: electrons, protons and neutrons. Electrons are negatively charge , protons are positively charge and neutrons is neutral. Protons and neutrons are located inside the nucleus, electrons are located outside the nucleus. Democritus’s Atomic Philosophy and Dalton’s Atomic Theory. The radii of most atoms is 5 × 10−11 m to 2 × 10−10 m.