Atom. Artist's impression of a pair of cupped hands holding an atom which is glowing with energy. The atom represented is beryllium. The blue sphere at centre is the nucleus which contains the positively-charged protons and neutral neutrons. Four negatively- charged electrons are travelling in circular orbits around the nucleus. Circular electron orbits were first proposed by the Danish physicist Niels Bohr in 1913. The hypothesis has since been replaced by quantum theory but still remains a useful visualisation of an atom's structure. An actual atom is so small that a single drop of water contains over a thousand billion billion of them.
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