The modern theory of atomic structure was first proposed by the Japanese physicist Hantaro Nagaoka (1865–1950) in 1904. In his model, electrons rotated in rings around a small central nucleus. In 1911, Ernest Rutherford discovered further evidence to prove that the nucleus of the atom is extremely small and dense and is surrounded by a much larger and less dense cloud of electrons. In 1913, the Danish physicist Niels Bohr (1885–1962) proposed a model that is known as the Bohr atom. It suggested that electrons orbit the nucleus in concentric quantum shells at certain well-specified distances from the nucleus corresponding to the electron’s energy levels. These orbits are known as “Bohr orbits.” Several years later, Erwin Schrödinger (1887–1961) proposed the Schrödinger wave equation, which provided a firm theoretical basis for the Bohr orbits.