HOW THE ATOM WORKS

EVERY atom, in everything around us, is like a miniature solar system. At its core is a nucleus, a cluster of particles which cling to each other like so many raisins pressed together in a hand. The nucleus is so tiny that it makes up only about a hundred-thousandth of the size of the entire atom, but so dense that it accounts for almost all its mass. Around it orbit its "planets", other particles called electrons.

Some of the particles in the nucleus carry a positive electrical charge, and are called protons. Each element has a different number of them: an atom of hydrogen has just one; an atom of oxygen has eight; an atom of uranium, 92. These protons are always matched by exactly the same number of orbiting electrons, which are negatively charged and so balance them out.

The rest of the cluster of particles in the nucleus carry no electrical charge and are called neutrons. While each atom of a particular element always has the same number of protons, the number of neutrons may differ: they do so in different varieties of the same element called isotopes.

Isotopes are distinguished by adding up the total particles in their nuclei. So uranium-238 has 92 protons and 146 neutrons, while uranium- 235 has the same 92 protons but 143 neutrons. The atoms thus characterised get yet another name, nuclides.

Some nuclides are stable, leading uneventful, unchanging lives. Others are not, and give vent to their instability by constantly trying to become something else. The particles in the nucleus of a uranium-238 atom, for example, can only just cling together. Eventually a chunk of four of them (two neutrons and two protons) will break away and the atom will turn into thorium-234 - with 90 protons and 144 neutrons.

But thorium-234 is unstable too. It also transforms itself - but using a different process. One of its neutrons turns into a proton and it becomes protactinium-234, with 91 protons and 143 neutrons. One of the orbiting electrons loses its partner as the proton changes its form, and so breaks away. Protactinium-234 is itself extremely unstable and loses no time in changing its own shape, and so, by one of these means after another, the atom goes on transforming itself and shedding particles until it ends up as lead. There are, of course, different such sequences for different nuclides.

All this is radioactivity and, as each change takes place, energy is released as radiation. The emission of a chunk of two protons and two neutrons, as from uranium-238, is called alpha radiation. The emission of an electron, as from thorium-234 is beta radiation. Often the unstable nuclide will be so excited that even such emissions of particles will not be enough to calm it down: it then gives off a vigorous burst of pure energy, gamma radiation. x-rays are similar to gamma radiation, but are created artificially by machines.

These different forms of radiation are emitted with different energies and penetrating power and so pose different kinds of danger. Alpha radiation is a very blunt instrument. The heavy chunk of neutrons and protons does great damage to any living cells it hits, but can be stopped by a sheet of paper, and can hardly penetrate the dead outer layers of the skin. So an alpha-emitting radioactive substance can be held safely in the hand, with just a little shielding, but is extremely dangerous if swallowed or breathed in.

Beta particles are more piercing, and can go through a centimetre or two of living tissue. And gamma rays, which travel at the speed of light, are extremely penetrating and will go through anything short of a thick slab of lead or concrete.