## Flexi Says: In 1800, John Dalton re-introduced the atom. He provided evidence for atoms and developed atomic theory. His theory is essentially correct. However, he incorrectly thought that atoms are the smallest particles of matter. In 1897, J.J. Thomson discovered electrons. He proposed the plum pudding model of the atom. In this model, negative electrons are scattered throughout a "sea" of positive charge. In 1911, Ernest Rutherford discovered the nucleus. He later discovered protons as well. Rutherford thought that electrons randomly orbit the nucleus. In 1915, Bohr proposed a new model of the atom in which electrons exist in states of constant energy, called stationary states, orbiting at fixed distances from the nucleus. Bohr’s work was primarily based on the emission spectrum of hydrogen. DeBroglie proposed in 1924 that any object exhibits a wavelength that is inversely proportional to its momentum. Because of this relationship, only very tiny particles will exhibit measurable wavelengths. Heisenberg showed that it is impossible to determine simultaneously both the exact location and exact velocity of an electron or any other particle. This became known as the Heisenberg uncertainty principle. Two years after DeBroglie’s work, in 1926, the Austrian physicist Erwin Schrӧdinger described the behavior of electrons in atoms as standing waves. The Schrödinger wave equation replaced the Bohr ideas about electron location with an uncertainty factor. The location of the electron can only be given as a probability that the electron is somewhere in a certain area.