1. The size scale of atoms is ∼ 1 angstrom (Å) = 0.1 nm = 10**(-10) m.

  2. The size scale of atomic nuclei is ∼ 1 fermi (fm) = 10**(-15) m = 10**(-5) Å.

    So nuclei are order of magnitude 10**5 times smaller than atoms.

    But nuclei have typically ∼ 2000 to 5000 times more mass than the electrons which set the size scale of the atoms.

    So overwhelmingly, the mass of atoms is in their nuclei.

  3. The grey shading represents the varying electron density (darker for more dense) or, as yours truly likes to call it, density of existence of the single electron. Most people use the expression probability density.

    What is density of existence? Quantum mechanics dictates that particles do NOT exist at one point, but are spread out in a continuum superposition of positions. The fractional amount of any particle at any point is the density of existence. The density of existence itself is determined from the wave function of the particle. The spread-out nature of particle existence is one of the things that makes quantum mechanics so tricky.

    The blue dot with the minus sign is just a schematic representation of the single electron.

  4. The single proton that is the nucleus is also spread out in a density of existence, but a much smaller one in scale: of order 10**5 times smaller in scale than that of the electron as follows from the discussion above.

    The red dot with the plus sign is just a schematic representation of the single proton.

  5. The dots are not-to-scale. If they were to-scale, we would NOT see them in the cartoon.

    The proton as follows from the discussion above of order 10**5 smaller than the whole atom. The electron may have zero size in terms of its structure NOT counting its being spread out into a density of existence.

  6. The proton and electron are bound together by the electrostatic force (AKA Coulomb's law force). Recall unlike electric charges attract; like electric charges repel.

    What holds a proton together? The strong nuclear force holds its constituent quarks together.

    What holds an electron together? Who knows. Electrons are currently thought of as truly elementary particles.

  7. What is actually the color of free electrons and free protons (i.e., electrons and protons NOT bound in larger structures: e.g., atoms, molecules, solids).

    In fact, free electrons and protons are reflective via the process of Thomson scattering. They reflect with only a small change in frequency for all electromagnetic radiation (EMR) with frequency much less than gamma rays (e.g., the visible band (fiducial range 0.4--0.7 μm)). So free electrons and protons are shiny. However, it seems hard to see them looking shiny in an ordinary sense. We certainly do see visible light reflected by free electrons and protons in some contexts. For example, some visible light emerging from the solar corona is Thomson scattered by free electrons and, to a much lesser extent, protons. But that visible light is just white light.

    What is actually the color of free neutrons? Yours truly CANNOT find an explicit answer, but suspects they are reflective too for all electromagnetic radiation (EMR) with frequency much less than gamma rays.????? They do NOT seem to have the structure to change the frequency of such EMR. However, their cross section for scattering might very small.?????