You could fit two billion atoms on the full stop at the end of this sentence. Scientists once thought atoms were like tiny hard balls that could never be split up, destroyed or created. In fact they are more like clouds of energy than hard balls. Indeed, they are mostly empty space dotted with a few even tinier particles, called sub-atomic particles. Right in the centre of every atom is a dense nucleus containing two kinds of particle, protons and neutrons.
Protons have a positive electrical charge while neutrons have none.
Around the nucleus are tinier, negatively-charged particles called electrons, travelling as fast as the speed of light. Most atoms have identical numbers of protons and electrons, so the charges balance each other. Atoms can be split up, but they are usually held firmly together by three forces - the strong electrical attraction between the negative electrons and the positive protons, and the strong and weak nuclear forces that bind the particles of the nucleus together. These forces, with gravity, are the basic forces that hold the universe together. The character of an atom depends on how many protons it has.