Atomic structure. Computer artwork of an atom of beryllium (chemical symbol Be). Its nucleus (red sphere) contains four positively-charged protons and five neutral neutrons. Four negatively-charged electrons are travelling in circular orbits around the nucleus. The charges of the protons and electrons cancel each other out so that the atom is neutral overall. Circular electron orbits were first proposed by the Danish physicist Niels Bohr in 1913. Scientists now believe that electron orbits are more eccentric and electrons inhabit a cloud of probability", but Bohr orbits remain a useful visualisation of an atom's structure."