Rutherford's model was quite simple, but it was later disproved because electrons have charge, and they should be attracted to the positively charged nucleus. In 1913, Niels Bohr added 'energy levels' to the atomic model. Electrons don't fall into the nucleus because they are contained in energy levels, and to change to higher energy levels extra energy is needed, and to change to lower energy levels a release of energy is needed. It is not possible to change energy states without changing the energy of the electron. If an electron gets hit by a photon (a particle that carries electromagnetic radiation) it will gain extra energy and go into a higher energy level (it changes states), then it will jump back down to a lower energy level, releasing its contained energy. This new model was called the Bohr model or the Rutherford-Bohr model. This added a whole new branch of science: Quantum physics.