The Magic of Reality by Richard Dawkins Page B

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    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