The ancient Greeks hypothesized that all physical structures were made up of smaller particles too small to detect with the human eye. In the early 19th century, scientist John Dalton refined the Greek model and defined atoms as the fundamental building blocks of nature. Nearly a century later, J.J. Thomson discovered that atoms contained electrons. He theorized that negatively charged electrons and other positively charged particles floated around inside an atom’s hard shell, like raisins in “plum pudding.” In 1911, Ernest Rutherford argued that an atom’s positive charge must be a result of an atomic nucleus and that electrons orbit the nucleus like planets around the sun. Just a few years later, Niels Bohr expanded Rutherford’s model introduced the idea that electrons orbit the atomic nucleus at varying energy levels. Bohr’s ideas inform the work of today’s scientists as they study the mysteries of quantum mechanics and how to harness nuclear energy.