The Science of Discworld Revised Edition by Terry Pratchett

Oxygen (15.9994), Fluorine (18.998403), Neon (20.179), Sodium (22.98977). A striking feature is that the atomic weight is nearly always close to a whole number, the first exception being chlorine at 35.453. All a bit puzzling, but it was an excellent start because now people could look for other patterns and relate them to atomic weights. However, looking for patterns proved easier than finding any. The list of elements was unstructured, almost random in its properties. Mercury, the only element known to be liquid at room temperature, was a metal. (Later just one further liquid was added to the list: bromine.) There were lots of other metals like iron, copper, silver, gold, zinc, tin, each a solid and each quite different from the others; sulphur and carbon were solid but not metallic; quite a few elements were gases. So unstructured did the list of elements seem that when a few mavericks – Johann Döbereiner, Alexandre-Emile Béguyrer de Chancourtois, John Newlands – suggested there might be some kind of order dimly visible amid the muddle and mess, they were howled down.
    Credit for coming up with a scheme that was basically
right
goes to Dimitri Mendeleev, who finished the first of a lengthy series of ‘periodic charts’ in 1869. His chart included 63 known elements placed in order of atomic weight. It left gaps where undiscovered elements allegedly remained to be inserted. It was ‘periodic’ in the sense that the properties of the elements started to repeat after a certain number of steps – the commonest being eight.
    According to Mendeleev, the elements fall into families, whose members are separated by the aforementioned periods, and in each family there are systematic resemblances of physical and chemical properties. Indeed those properties vary so systematically as you run through the family that you can see clear, though not always exact, numerical patterns and progressions. The scheme works best, however, if you assume that a few elements are missing from the known list, hence the gaps. As a bonus, you can make use of those family resemblances to
predict
the properties of those missing elements before anybody finds them. If those predictions turn out to be correct when the missing elements are found – bingo. Mendeleev’s scheme still gets modified slightly from time to time, but its main features survive: today we call it the Periodic Table of the Elements.
    We now know that there is a good reason for the periodic structure that Mendeleev uncovered. It stems from the fact that atoms are not as indivisible as Democritus and Boyle thought. True, they can’t be divided
chemically
– you can’t separate an atom into component pieces by doing chemistry in a test tube – but you can ‘split the atom’ with apparatus that is based on physics rather than chemistry. The ‘nuclear reactions’ involved require much higher energy levels – per atom – than you need for chemical reactions, which is why the old-time alchemists never managed to turn lead into gold. Today, this could be done – but the cost of equipment would be enormous, and the amount of gold produced would be extremely small, so the scientists would be very much like Discworld’s own alchemists, who have only found ways of turning gold into less gold.
    Thanks to the efforts of the physicists, we now know that atoms are made from other, smaller particles. For a while it was thought that there were just three such particles: the neutron, the proton, and the electron. The neutron and proton have almost equal masses, while the electron is tiny in comparison; the neutron has no electrical charge, the proton has a positive charge, and the electron has a negative charge exactly opposite to that of the proton. Atoms have no overall charge, so the numbers of protons and electrons are equal. There is no such restriction on the number of neutrons. To a good approximation, you get an element’s atomic weight by adding up the numbers of protons and