Atomic Structure
Atoms are so tiny it's not possible to see them even with the most advanced microscopes. In fact more than a million would fit into the thickness of a sheet of paper. Although atoms are the smallest units that can exist under normal conditions, high energy physics can smash atoms into their much smaller component parts. Today atoms are believed to consist mainly of space, with a dense nucleus at their centre. Each nucleus is made of protons which are positively-charged and neutrons which have no charge. Electrons, which are negatively-charged circulate, around the nucleus in fixed orbits a bit like planets revolving round the Sun. Because the number of protons in any atom is always equal to the number of electrons, atoms have no charge. For example a hydrogen atom consists of one proton and one electron. A carbon atom consists of 6 protons, 6 electrons and also 6 neutrons. For a particular element the number of protons and electrons is fixed and always the same however it is possible to find different numbers of neutrons in the nucleus. For instance some hydrogen atoms in nature have one neutron. These are atoms are known as deuterium or heavy hydrogen. Some carbon atoms have 7 neutrons (carbon 13, C13) and some have 8 neutrons (carbon 14, C14) These variations are known as isotopes. Some isotopes are radioactive which means they naturally break down or decay over a specific period of time giving off radiation. This can have useful practical applications. For example carbon 14 is used to carbon date archeological artifacts.