Simple Nature - Light and Matter

For example they could easily strip off one or two electrons fromhelium, making He + or He ++ , but nobody could make He +++ , presumablybecause the nuclear charge of helium was only +2e. Unfortunatelyonly a few of the lightest elements could be stripped completely,because the more electrons were stripped off, the greater thepositive net charge remaining, and the more strongly the rest of thenegatively charged electrons would be held on. The heavy elements’atomic numbers could only be roughly extrapolated from the lightelements, where the atomic number was about half the atom’s massexpressed in units of the mass of a hydrogen atom. Gold, for example,had a mass about 197 times that of hydrogen, so its atomicnumber was estimated to be about half that, or somewhere around100. We now know it to be 79.How did we finally find out? The riddle of the nuclear chargeswas at last successfully attacked using two different techniques,which gave consistent results. One set of experiments, involvingx-rays, was performed by the young Henry Mosely, whose scientificbrilliance was soon to be sacrificed in a battle between European imperialistsover who would own the Dardanelles, during that pointlessconflict then known as the War to End All Wars, and now referredto as World War I.k / An alpha particle has to comemuch closer to the low-chargedcopper nucleus in order to be deflectedthrough the same angle.Since Mosely’s analysis requires several concepts with which youare not yet familiar, we will instead describe the technique usedby James Chadwick at around the same time. An added bonus ofdescribing Chadwick’s experiments is that they presaged the importantmodern technique of studying collisions of subatomic particles.In grad school, I worked with a professor whose thesis adviser’s thesisadviser was Chadwick, and he related some interesting storiesabout the man. Chadwick was apparently a little nutty and a completefanatic about science, to the extent that when he was held in aGerman prison camp during World War II, he managed to cajole hiscaptors into allowing him to scrounge up parts from broken radios482 Chapter 8 Atoms and Electromagnetism