 In this segment, we developed the basic quantum mechanics for electrons around the atom and measured the size of atomic components. At the end of our previous segment, we used a scanning electron microscope to see carbon atoms, 14-hundredths of a nanometer in diameter. Using Rutherford scattering techniques covered in this segment, we measured the size of a proton at 1.76 millionths of a nanometer. That's 20,000 times smaller than the atom. At this scale, we find that the neutron is about the same size and mass as the proton. Also in this segment, we added spin as an intrinsic property of particles to go along with mass and electric charge. Protons and neutrons both display the same spin properties as electrons when they traverse the Stern-Gerlach apparatus. So their spin is one-half as well. The notable difference between these particles is that the proton has a positive charge with the same magnitude as the electron's negative charge, but the neutron is neutral with no charge at all. For electrons, it's hard to talk about their size because their wave packets are different for varying circumstances from standing waves in thin shells to scattered waves in electron microscopes. What we did in this segment was to calculate its length around the hydrogen nucleus at 0.033 nanometers. For photons, we see that they have no mass at all, no charge, and a spin equal to one. For the gamma rays coming out of uranium, the wavelength is one one-hundredth of a nanometer. That's 51,000 times smaller than the wavelength of green light. Looking at the atom's nucleus, we see one main question. How do positively charged protons pack together in a nucleus when their repulsive positive charges would have them flying apart? We'll make some headway on this question in our next segment on elementary particles.