The Magic of Reality (8 page)

An early model of the atom was the so called ‘currant bun’ model proposed by the great English physicist J. J. Thomson at the end of the nineteenth century. I won’t describe it because it was replaced by the more successful Rutherford model, first proposed by the same Ernest Rutherford who split the atom, who came from New Zealand to England to work as Thomson’s pupil and who succeeded Thomson as Cambridge’s Professor of Physics. The Rutherford model, later refined in turn by Rutherford’s pupil, the celebrated Danish physicist Niels Bohr, treats the atom as a tiny, miniaturized solar system. There is a nucleus in the middle of the atom, which contains the bulk of its material. And there are tiny particles called electrons whizzing around the nucleus in ‘orbit’ (though ‘orbit’ may be misleading if you think of it as just like a planet orbiting the sun, because an electron is not a little round thing in a definite place).

One surprising thing about the Rutherford / Bohr model, which probably reflects a real truth, is that the distance between each nucleus and the next is very large compared with the size of the nuclei, even in a hard chunk of solid
matter
like a diamond. The nuclei are hugely spaced out. This is the point I promised to return to.

Remember I said that a diamond crystal is a giant molecule made of carbon atoms like soldiers on parade, but a parade in three dimensions? Well, we can now improve our ‘model’ of the diamond crystal by giving it a scale – that is, a sense of how sizes and distances in it relate to one another. Suppose we represent the nucleus of each carbon atom in the crystal not by a soldier but by a football, with electrons in orbit around it. On this scale, the neighbouring footballs in the diamond would be more than 15 kilometres away.

The 15 kilometres between the footballs would contain the electrons in orbit around the nuclei. But each electron, on our ‘football’ scale, is much smaller than a gnat, and these miniature gnats are themselves several kilometres away from the footballs they are flying around. So you can see that – amazingly – even the legendarily hard diamond is almost entirely empty space!

The same is true of all rocks, no matter how hard and solid. It is true of iron and lead. It is also true of even the hardest wood. And it is true of you and me. I’ve said that solid matter is made of atoms ‘packed’ together, but ‘packed’ means something rather odd here because the atoms themselves are mostly empty space. The nuclei of the atoms are spaced out so far apart that, if they were scaled up to footballs, any pair of them would be 15 kilometres apart with only a few gnats in between.

How can this be? If a rock is almost entirely empty space, with the actual matter dotted about like footballs
separated
by kilometres from their neighbours, how come it feels so hard and solid? Why doesn’t it collapse like a house of cards when you sit on it? Why can’t we see right through it? If both a wall and I are mostly empty space, why can’t I walk straight through the wall? Why do rocks and walls feel hard, and why can’t we merge our spaces with theirs?

We have to realize that what we feel and see as solid matter is more than just nuclei and electrons – the ‘footballs’ and the ‘gnats’. Scientists talk about ‘forces’ and ‘bonds’ and ‘fields’, which act in their different ways both to keep the ‘footballs’ apart and to keep the components of each ‘football’ together. And it is those forces and fields that make things feel solid.

When you get down to really small things like atoms and nuclei, the distinction between ‘matter’ and ‘empty space’ starts to lose its meaning. It isn’t really right to say that the nucleus is ‘matter’ like a football, and that there is ‘empty space’ until the next nucleus.

We define solid matter as ‘what you can’t walk through’. You can’t walk through a wall because of these mysterious forces that link the nuclei to their neighbours in a fixed position. That’s what solid means.

Liquid means something similar, except that the mysterious fields and forces hold the atoms together less tightly, so they slide over each other, which means that you can walk through water, although not so fast as you can walk through air. Air, being a gas (a mixture of gases, actually), is easy to walk through because the atoms in a gas whizz about freely, rather than being tied to each other. A gas becomes
difficult
to walk through only if most of the atoms are whizzing in the same direction, and it is the opposite direction to the one in which you are trying to walk. This is what happens when you are trying to walk against the wind (that’s what ‘wind’ means). It can be difficult to walk against a strong gale, and impossible against a hurricane or against the artificial gale hurled out behind a jet engine.

We can’t walk through solid matter, but some very small particles such as the ones called photons can. Light beams are streams of photons, and they can go right through some kinds of solid matter – the kinds we call ‘transparent’. Something about the way the ‘footballs’ are arranged in glass or in water or in certain gemstones means that photons can pass right between them, although they are slowed down a bit, just as you are slowed down when you try to walk through water.

With a few exceptions like quartz crystals, rocks aren’t transparent, and photons can’t pass through them. Instead, depending on the rock’s colour, they are either absorbed by the rock or reflected from its surface, and the same is true of most other solid things. A few solid things reflect photons in a very special straight-line way, and we call them mirrors. But most solid things absorb many of the photons (they aren’t transparent), and scatter even the ones that they reflect (they don’t behave like mirrors). We just see them as ‘opaque’, and we also see them as having a colour, which depends on which kinds of photons they absorb and which kinds they reflect. I’ll return to the important subject of colour in Chapter 7, ‘What is a Rainbow?’ Meanwhile, we need to shrink our vision to the
very
small indeed, and look right inside the nucleus – the football – itself.

The tiniest things of all

The nucleus isn’t really like a football. That was just a crude model. It certainly isn’t round like a football. It isn’t even clear whether we should speak of it as having a ‘shape’ at all.

Maybe the very word ‘shape’, like the word ‘solid’, loses all meaning down at these very tiny sizes. And we are talking very very tiny indeed: the full stop at the end of this sentence contains about a million million atoms of printing ink.

Each nucleus contains smaller particles called protons and neutrons. You can think of them as balls too, if you wish, but like the nuclei they are not really balls. Protons and neutrons are approximately the same size as each other. They are very very tiny indeed, but even so they are still 1,000 times bigger than the electrons (‘gnats’) in orbit around the nucleus. The main difference between a proton and a neutron is that the proton has an electric charge. Electrons, too, have an electric charge, opposite to that of protons. We needn’t bother with exactly what ‘electric charge’ means here. Neutrons have no charge.

Because electrons are so very very very tiny (while protons and neutrons are only very very tiny!) the mass of an atom is, to all intents and purposes, just its protons and neutrons. What does ‘mass’ mean? Well, you can think of mass as rather like weight, and you can measure it using the same units as weight (grams or pounds). Weight is not the same as mass, however, and I’ll need to explain the difference, but I’m
postponing
that to the next chapter. For the moment just think of ‘mass’ as something like ‘weight’.

The mass of an object depends almost entirely on how many protons and neutrons it has in all its atoms added together. The number of protons in the nucleus of any atom of a particular element is always the same, and is equal to the number of electrons in orbit around the nucleus, although the electrons don’t contribute noticeably to the mass because they are too small. A hydrogen atom has only one proton (and one electron). A uranium atom has 92 protons. Lead has 82. Carbon has 6. For every possible number from 1 to 100 (and a few more), there is one and only one element that has that number of protons (and the same number of electrons). I won’t list them all, but it would be easy to do so.

The number of protons (or electrons) that an element possesses is called the ‘atomic number’ of that element. So you can define an element not just by its name but by its own unique atomic number. For example, element number 6 is carbon; element number 82 is lead. The elements are conveniently set out in a table called the periodic table – I won’t go into why it’s called that, although it is interesting. But now is the moment to return, as I promised I would, to the question of why, when you cut a piece of, say, lead into smaller and smaller pieces, you eventually reach a point where, if you cut it again, it is no longer lead. An atom of lead has 82 protons. If you split the atom so that it no longer has 82 protons it ceases to be lead.

The number of neutrons in an atom’s nucleus is less fixed than the number of protons: many elements have
different
versions, called isotopes, with different numbers of neutrons. For example, there are three isotopes of carbon, called Carbon-12, Carbon-13 and Carbon-14. The numbers refer to the mass of the atom, which is the sum of the protons and neutrons. Each of the three has six protons. Carbon-12 has six neutrons, Carbon-13 has seven neutrons and Carbon-14 has eight neutrons. Some isotopes, for example Carbon-14, are radioactive, which means they change into other elements at a predictable rate, although at unpredictable moments. Scientists can use this feature to help them calculate the age of fossils. Carbon-14 is used to date things younger than most fossils, for example ancient wooden ships.

Well then, does our quest to cut things ever smaller and smaller end with these three particles: electrons, protons and neutrons? No – even protons and neutrons have an inside. Even they contain yet smaller things, called quarks. But that is something I’m not going to talk about in this book. That’s not because I think you wouldn’t understand it. It is because I know
I
don’t understand it! We are here moving into a wonderland of the mysterious. And it is important to recognize when we reach the limits of what we understand. It is not that we shall never understand these things. Probably we shall, and scientists are working on them with every hope of success. But we have to know what we don’t understand, and admit it to ourselves, before we can begin to work on it. There are scientists who understand at least something of this wonderland of the very small, but I am not one of them. I know my limitations.

Carbon – the scaffolding of life

All the elements are special in their different ways. But one element, carbon, is so special that we will end the chapter by talking briefly about that. Carbon chemistry even has its own name, separating it from the whole of the rest of chemistry: ‘organic’ chemistry. All the rest of chemistry is ‘inorganic’ chemistry. So what is so special about carbon?

The answer is that carbon atoms link up with other carbon atoms to form chains. The chemical compound octane, which, as you may know, is an ingredient of petrol (gasoline), is a rather short chain of eight carbon atoms with hydrogen atoms sticking out to the sides. The wonderful thing about carbon is that it can make chains of any length, some literally hundreds of carbon atoms long. Sometimes the chains come around in a loop. For example, molecules of naphthalene (the substance that mothballs are made of) are also made of carbon with hydrogen attached, this time in two loops.

Carbon chemistry is rather like the toy construction kit called Tinkertoy. In the laboratory, chemists have succeeded in making carbon atoms join up with each other, not just in simple loops but in wonderfully shaped Tinkertoy-like molecules nicknamed Buckyballs and Buckytubes. ‘Bucky’ was the nickname of Buckminster Fuller, the great American architect who invented the geodesic dome. The Buckyballs and Buckytubes scientists have made are artificial molecules. But they show the Tinkertoyish way in which carbon atoms can be joined together into scaffolding-like structures that can be indefinitely large. (Just recently the exciting news was
announced
that Buckyballs have been detected in outer space, in the dust drifting near to a distant star.) Carbon chemistry offers a near-infinite number of possible molecules, all of different shapes, and thousands of different ones are found in living bodies.

One very large molecule called myoglobin, for example, is found, in millions of copies, in all our muscles. Not all the atoms in myoglobin are carbon atoms, but it is the carbon atoms that join together in these fascinating Tinkertoy-like scaffolding structures. And that is really what makes life possible. When you think that myoglobin is only one example among thousands of equally complicated molecules in living cells, you can perhaps imagine that, just as you can build pretty much anything you like if you have a large enough Tinkertoy set, so the chemistry of carbon provides the vast range of possible forms required to put together anything so complicated as a living organism.

What, no myths?

This chapter has been unusual in that it didn’t begin with a list of myths. This was only because it was so hard to find any myths on this subject. Unlike, say, the sun, or the rainbow, or earthquakes, the fascinating world of the very small never came to the notice of primitive peoples. If you think about this for a minute, it’s not really surprising. They had no way of even knowing it was there, and so of course they didn’t invent any myths to explain it! It wasn’t until the microscope was invented in the sixteenth century that people discovered that ponds and lakes, soil and dust, even our own bodies, teem
with
tiny living creatures, too small to see, yet complicated and, in their own way, beautiful – or perhaps frightening, depending on how you think about them.

Dust mites are distantly related to spiders but too small to see except as tiny specks. There are thousands of them in every home, crawling through every carpet and every bed, quite probably including yours. If primitive peoples had known about them, you can imagine what myths and legends they might have invented to explain them! But before the invention of the microscope, their existence was not even dreamed of – and so there are no myths about them. And, small as it is, even a dust mite contains more than a hundred trillion atoms.