an atom is the smallest particle of an element, although nobody has ever seen an atom, its hypothetical structure fits experimental evidence that has been measured very exactly. our present planetary model of the atom was proposed by niels bohr in 1913 as shown in fig. 1-1 atom consists of a central nucleus surrounded by orbiting negatively charged particles called electrons. the nucleus consists of positively charged particles called protons and uncharged particles called neutrons. then the positive and negative charges are balanced, as the proton and electron have equal and opposite charges. a proton is 1840 times heavier than an electron