In chemical reactions, the atoms that make up the elements do not change. However, in some circumstances the atoms can be made to change (such as when two hydrogen atoms are made to fuse together to produce a helium atom) and some elements, such as uranium, continually change. Elements that continually change are called radioactive elements.

All atomic change is characterised by a release of energy, which may be in the form of heat alone, heat and light together, or, as in the case of the radioactive elements, heat, light and the release of "radiation" (rays or particles of matter).

All the elements with an atomic number of 88 and above in the Periodic Table are radioactive. In addition, some common elements (such as hydrogen, oxygen and carbon) have radioactive forms called isotopes.

The changes in radioactive elements take place in the core, or nucleus, of the element's atoms. This is why scientists call them nuclear reactions and why we use such terms as nuclear energy and nuclear bombs.

Nuclear reactions are often far more powerful (they release much more energy) than chemical reactions. This is why they have been used in bombs as well as in power stations. However, when properly handled, they are not any more dangerous than chemical reactions.

Radioactivity ­ energy given out by a radioactive substance ­ is a result of changes to the nucleus of an atom. The atoms of the radioactive elements send out, or radiate, particles and waves of energy from their core (nucleus) without any form of chemical reaction.

Radioactivity is as old as the Universe, yet it was only discovered quite recently. Radium, the first radioactive element to be discovered, was only noticed in 1896 when French scientist Antoine Henri Becquerel accidentally left an unexposed photographic plate in a drawer next to a piece of ore which contained radium. When he developed the plate he found that it had been "fogged" by the radioactivity from the radium. The word radioactivity was suggested in the early years of this century by Pierre and Marie Curie, who did much of the pioneering work on radioactive elements.

Radioactive elements are not stable like other elements. Because they continuously give out radiation, they are also
always changing. Although radium and uranium are probably the best known of the radioactive elements, there are many others with very useful properties.

Uranium

The element uranium, chemical symbol U, is named for the planet Uranus, since the planet was discovered in 1781, just eight years before the element uranium.

Uranium, a silvery-white, extremely dense metal, was first discovered in the mineral pitchblende. Uranium is not an especially rare mineral. It is more plentiful than, say, mercury or silver. However, it has become of vital importance in the nuclear age.

An overwhelmingly large proportion of uranium on Earth is uranium-238. This makes it the heaviest atom commonly found in nature. Uranium turns blue in air because it develops an oxide coating.

Uranium is not found in concentrated form; many tonnes of ore have to be processed to obtain even one gram of the element.

The biggest deposits of uranium ore are at Blind River, Canada, in South Africa, Australia, France, and in Colorado and Utah in the United States.

The most important compound of uranium is uranium hexafluoride gas, which can be used to separate uranium-238 from uranium-235, the main ingredient of the atomic bomb.

Inside the elements

Chemistry is concerned with the exchange or sharing of electrons in the outside regions of an atom, rather than with what happens in the core of an atom, in the nucleus. However, when working with radioactive elements, scientists, need to understand the nature of the particles that make up the atoms.

Radioactive elements vary greatly. All are quite rare; most are metals. But radioactive isotopes cannot be used in the same way as other metals ­ to make new materials. Indeed, if used this way they could make rather dangerous materials. Rather, radioactive elements are used for their radioactive properties alone. of neutrons they contain. Each of these variations is called an isotope (meaning "the same but different").

Because like charges repel each other, there is always a force trying to push the protons apart. Provided there are not too many protons in the nucleus, other forces can hold the protons together. But if the ratio of protons to neutrons is not within certain limits, protons may not be held firmly together, and they form an unstable nucleus. This is what makes isotopes of some elements radioactive.

For example, carbon, the element found in all living things has the chemical symbol C. The normal form (isotope) has an atomic weight of 12 and is written ¹²C or carbon-12, but the radioactive version (the radioactive isotope) has two extra neutrons, so its symbol is ¹⁴C (carbon-14). As we shall see, the radioactive form behaves chemically just like the non-radioactive form, although one will never change into the other.
What makes an element radioactive?

Inside an atom there are three kinds of particle: protons, neutrons and electrons. The nucleus, the tiny
core of the atom, contains protons (positively charged particles) and neutrons (so called because they are neutral and have no charge). The region beyond the nucleus contains (negatively charged) electrons that balance out the charge of the protons. The electrons are usually thought of as orbiting the tiny nucleus, like planets orbiting the Sun.

As you would expect, properties of an atom give rise to some of the most important properties of an element. For example, the atomic number of an element is equal to the total number of protons in an atom (the atomic number
of each element is given on pages 46­47). The total number of protons and neutrons gives the atomic weight (mass). There are roughly as many protons
as neutrons, which is why the atomic weight is about (but not exactly) twice the atomic number.

In some cases several versions of the same element occur, identical in all their chemical properties and varying only in the number

Properties of uranium

Harmful to life
Heavy element
Melting point 1132°C
A soft, silvery metal,
chemical symbol U
Decays naturally through a radioactivity series to lead
Used as a source of fissionable fuel
Poor conductor of electricity
Dissolves in acids
Atomic number 92, atomic weight about 238

Also

The origin of the elements

Why do we have so many different elements and where did they form? This is a question that a nuclear scientist is best able to answer, because the answer lies in the core, or nucleus, of atoms.

At the beginning of time (the instant of the creation of the known Universe, called the Big Bang) the only element in existence was hydrogen. All the other elements are, in some way or another, "daughters" of hydrogen. So, for example, hydrogen atoms are forced together (fused) to make helium and so on down a long line. In this line carbon, for example, is transformed into oxygen. Together the elements created by fusion make the stuff of life.

Nuclear reactions are the most fundamental of all reactions, creating the elements themselves. They are going on today, just as they have
been happening since the Big Bang. Most nuclear activity occurs in the stars, although a small amount is happening in the rocks of the Earth. But even nuclear reactions on a small scale are noticeable because of the outpouring of energy ­ radiation ­ that accompanies any nuclear change.

Changing one element into another

The change from one element to another is called transmutation. Remember that atoms are made up of different combinations of protons, neutrons and electrons, so the break up of a neutron leaves behind an atom with
a different combination of neutrons, protons and electrons.

This new combination is a new element. So when uranium loses nuclear particles, a whole chain of events is set in motion until finally lead is created. Different forms of radiation are produced during this process.

The chain of events tends to happen very slowly, sometimes only over many millions of years. As we shall see, this slow rate of change gives rise to one of the main problems of dealing with radioactive materials.

In nature, new radioactive materials are produced all the time. This is because particles from space (known as cosmic rays) are continually bombarding the air and the planet's surface (including our bodies). The particles they contain have enough energy to change nitrogen-14 in the atmosphere to carbon-14, which is absorbed in living tissue in the same way as carbon-12. For this reason, carbon-14 can be used as a radioactive "clock" .