Free The Magic of Reality by Richard Dawkins

too small to be cut any smaller, and that is where our word ‘atom’ comes from. An atom of gold is the smallest possible bit of gold. Even if it were possible to cut it any smaller, it would cease to be gold. An atom of iron is the smallest possible bit of iron. And so on.
    We now know that there are about 100 different kinds of atoms, of which only about 90 occur in nature. The few others have been concocted by scientists in the lab, but only in tiny quantities.
    Pure substances that consist of one kind of atom only are called elements (same word as was once used for earth, air, fire and water, but with a very different meaning). Examples of elements are hydrogen, oxygen, iron, chlorine, copper, sodium, gold, carbon, mercury and nitrogen. Some elements, such as molybdenum, are rare on Earth (which is why you may not have heard of molybdenum) but commoner elsewhere in the universe (if you wonder how we know this, wait for Chapter 8).
    Metals such as iron, lead, copper, zinc, tin and mercury are elements. So are gases such as oxygen, hydrogen, nitrogen and neon. But most of the substances that we see around us are not elements but compounds. A compound is what you get when two or more different atoms join together in a particular way. You’ve probably heard water referred to as ‘H 2 O’ . This is its chemical formula, and means it is a compound of one oxygen atom joined to two hydrogen atoms. A group of atoms joined together to make a compound is called a
molecule
. Some molecules are very simple: a molecule of water, for example, has just those three atoms. Other molecules, especially those in living bodies, have hundreds of atoms, all joined together in a very particular way. Indeed, it is the way they are joined together, as well as the type and number of atoms, that makes any particular molecule one compound and not another.
    You can also use the word ‘molecule’ to describe what you get when two or more of the same kind of atom join together. A molecule of oxygen, the gas we need in order to breathe, consists of two oxygen atoms joined together. Sometimes three oxygen atoms join together to form a different kind of molecule called ozone. The number of atoms in a molecule really makes a difference, even if the atoms are all the same.
    Ozone is harmful to breathe, but we benefit from a layer of it in the Earth’s upper atmosphere, which protects us from the most damaging of the sun’s rays. One of the reasons Australians have to be especially careful when sunbathing is that there is a ‘hole’ in the ozone layer in the far south.
    Crystals – atoms on parade
    A diamond crystal is a huge molecule, of no fixed size, consisting of millions of atoms of the element carbon stuck together, all lined up in a very particular way. They are so regularly spaced inside the crystal, you could think of them as being like soldiers on parade, except that they are parading in three dimensions, like a shoal of fish. But the number of ‘fish’ in the shoal – the number of carbon atoms in even the smallest diamond crystal – is gigantic, more than all the fish (plus all the people) in the world. And ‘stuck together’ is a misleading way to describe them if it makes you think of the atoms as solid lumps of carbon closely packed with no space in between. In fact, as we shall see, most ‘solid’ matter consists of empty space. That will take some explaining! I’ll come back to it.
    All crystals are built up in the same ‘soldiers-on-parade’ way, with atoms regularly spaced in a fixed pattern that gives the whole crystal its shape. Indeed, that is what we mean by a crystal. Some ‘soldiers’ are capable of ‘parading’ in more than one way, producing very different crystals. Carbon atoms, if they parade in one way, make the legendarily hard diamond crystals. But if they adopt a different formation they make crystals of graphite, so soft it is used as a lubricant.
    We think of crystals as beautiful transparent