Ok. In my leapfrog study of Chemistry over the last 40+ years, we didn't cover this in high school chemistry. I have often puzzled about orbitals but never asked about the mechanism which caused them. Now I am fresh with questions which brought me to the previous answer to my previous question, "how does a reduction of neutrons make an isotope more unstable?" I could understand more neutrons making the isotope radioactive (but I really don't). I was annotating my new Periodic Table of the Elements with group numbers, trends, and electron shell counts (not orbitals). I noticed that Uranium had an isotope with an atomic mass which was less than 238 and it was radioactive. I didn't understand how that could be. This morning I asked the question and it took me to Stable_and_Unstable_Isotopes in another course. When I looked for it again, I was in Allied Health. When I read further, I saw the reference to Quantum Physics of which my wife and I are interested. (We have an enlarged photo of the 1929 Solvay Conference over our piano in the family room.) Peace.

What other courses use this content? (Section 21.2, for example)