In 400 b. c., a greek philosopher named democritus theorized that matter is made of simple parts. he called these parts atoma, or atoms. in 1897, a british physicist named joseph thomson discovered electrons. he theorized that atoms were positively-charged spheres, and negatively-charged electrons were embedded in the sphere. a few years later, in 1913, another physicist named niels bohr theorized that atoms had a positively-charged nucleus, and negatively-charged electrons orbited the nucleus. this information demonstrates how