The Science of Discworld Revised Edition

naturally, the same as protons), with atomic weight 35. The other kind has 17 protons and 20 neutrons (and 17 electrons, again) – an extra two neutrons, which raises the atomic weight to 37. Naturally occurring chlorine is a mixture of these two ‘isotopes’, as they are called – in roughly the proportions 3 to 1. The two isotopes are (almost) indistinguishable chemically, because they have the same number and arrangement of electrons, and that’s what makes chemistry work; but they have different atomic physics.
    It is easy for a non-physicist to see why the wizards of UU considered this universe to be made in too much of a hurry out of obviously inferior components …
    Where did all those 113 elements come from? Were they always around, or did they get put together as the universe developed?
    In our Universe, there seem to be five different ways to make elements:
    • Start up a universe with a Big Bang, obtaining a highly energetic (‘hot’) sea of fundamental particles. Wait for it to cool (or possibly use one you made earlier …). Along with ordinary matter, you’ll probably get a lot of exotic objects like tiny Black Holes, and magnetic monopoles but these will disappear pretty quickly and only conventional matter will remain – mostly. In a very hot universe, electromagnetic forces are too weak to resist disruption, but once the universe is cool enough, fundamental particles can stick together as a result of electromagnetic attraction. The only element that arises directly in this manner is hydrogen – one electron joined with one proton. However, you get an awful lot of it: in our universe it is by far the commonest element, and nearly all of it arose from the Big Bang.
    Protons and electrons can also associate to form deuterium (one electron, one proton, one neutron) or tritium (one electron, one proton, two neutrons), but tritium is radioactive, meaning that it spits out neutrons and decays into hydrogen again. A far more stable product is helium (two electrons, two protons, two neutrons), and helium is the second most abundant element in the universe.
    • Let gravity get in on the act. Now hydrogen and helium collect together to form stars – the wizards’ ‘furnaces’. At the centre of stars, the pressure is extremely high. This brings new nuclear reactions into play, and you get nuclear fusion, in which atoms become so squashed together that they merge into a new, bigger atom. In this manner, many other familiar elements were formed, from carbon, nitrogen, oxygen, to the less familiar lithium, beryllium and so on up to iron. Many of these elements occur in living creatures, the most important being carbon. For reasons to do with its unique electron structure, carbon is the only atom that can combine with itself to form huge, complex molecules, without which our kind of life would be impossible. 1 Anyway, the point is that most of the atoms from which you are made must have come into being inside a star. As Joni Mitchell sang at Woodstock: 2 ‘We are stardust.’ Scientists like quoting this line, because it sounds as though they were young once.
    • Wait for some of the stars to explode. There are (comparatively) small explosions called novas, meaning ‘new (star)’, and more violent ones – supernovas. (What’s ‘new’ is that usually we can’t
see
the star until it explodes, and then we can.) It’s not just that the nuclear fuel gets used up: the hydrogen and helium that fuel the star fuse into heavier elements, which in effect become impurities that disturb the nuclear reaction. Pollution is a problem even at the heart of a star. The physics of these early suns changes, and some of the larger ones explode, generating higher elements like iodine, thorium, lead, uranium, and radium. These stars are called ‘Population II’ by astrophysicists – they are old stars, low in heavy elements, but not lacking them entirely.
    • There are two kinds of supernova, and the other type creates heavy elements in abundance, leading to ‘Population I’ stars, which are much younger than Population II. 3 Because many of these elements have unstable atoms, various other elements are made by their radioactive decay. These ‘secondhand’ elements include lead.
    • Lastly, human beings have made some elements by special arrangements in atomic reactors – the best known being plutonium, a by-product of conventional uranium reactors and a raw material for nuclear weapons. Some rather