I found a wonderful display about the physicist Henry Moseley in the History of Science Museum in Oxford. I'd not heard of him. He made important progress on elements and the Periodic Table using X-rays. The idea is that electrons are fired at an element with such high energy that they are able to dislodge electrons from the inner electron energy level. When they fall back to the level, they emit X-rays. The spectrum of such X-rays must be unique to each element in the same way as the spectrum of light from an excited gas vapour lamp is when I do this in class.
