Radioactivity is a form of energy given off by some types of atom with a high atomic number, such as uranium, radium and plutonium. Some forms of radiation consist of protons and neutrons bound together, while other forms consist of electrons or other particles called positrons. As radiation is emitted, the loss of particles from the original atoms changes their properties to those of another atom with a lower atomic number.
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Hydrogen is the lightest and simplest element, and its atomic number is 1. Uranium is the heaviest and its atomic number is 92 — indicating that it has 92 protons in each atom.
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The periodic table is a list of all the elements, arranged in such a way that elements with similar properties are grouped together. Each element in the table is given a number, called an atomic number, which indicates the number of protons the atom has. (A single atom has the same number of protons as electrons.) Elements with the same number of electrons in their outer shells are grouped together in the table.
How many different elements are there?
There are 92 elements that exist naturally, but it has been possible for scientists to create many more in the laboratory. These artificial elements are radioactive and they quickly decay or lose their radioactivity. Some exist for only a few seconds or less.
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Antimatter is like normal matter, but it is made up of particles that are exactly the opposite to those making up ordinary matter. A normal electron, for example, has a negative charge, but in anti-matter, the corresponding particle has a positive charge. Antimatter was first proposed in 1930, and it is now possible to make it in tiny amounts, using a machine called a particle accelerator. When antimatter and normal matter make contact, huge amounts of energy are released.
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Niels Bohr, a Danish physicist, was awarded the Nobel Prize in 1922 for his discovery of the structure of the atom. He went on to help develop the atomic bomb in 1943.
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The smallest part of a substance that can exist on its own is a molecule. This consists of anything between two and several thousand atoms, which are linked together by chemical bonds. Sometimes the molecule consists of identical atoms, and it is called an element. When a group of different types of atom is connected together in this way, it forms a compound.
The way in which the atoms become linked depends on the types of chemical bonds. This, in turn, determines the shape the molecule and its chemical properties.
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