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