 If I asked you to draw a model of the atom, you'd probably draw something like this. This diagram is often called the planetary model of the atom, and it was put forward by Ernest Rutherford in 1911. And it's got a lot going for it as far as models of the atoms go, but it's got a few drawbacks as well. On the pros side, this was the first model of the atom with a nucleus, a concentration of mass and positive charge in the center of the atom. Rutherford proposed this idea after his famous gold foil experiment, where he showed that alpha particles deflect off of something small, dense, and positive in the middle of an atom. But on the cons side is the way the electrons are shown. They're moving in circles. So what's wrong with circles? Well, the well-established physics principle put forward by James Clerk Maxwell is that any charged accelerating particle produces electromagnetic radiation, or EMR. So that means everything from x-rays to visible light and microwaves is produced by charged accelerating particles. And what we have here in the model of the atom is an electron, which is a charged particle, moving in a circular path. And circular objects are going to produce centripetal acceleration towards the center of the circle. So according to classical physics, the planetary model does not work, since an electron that constantly moves in circles would constantly admit EMR, lose energy, and spiral into the nucleus, which just doesn't happen to atoms in the world around us. So Rutherford was stuck, but luckily for us, the answer to this problem was working on a seemingly unrelated issue right next door. In 1913, Neil Bohr was tasked with a pretty innocent-sounding problem. Why do hot gases release different colors of light, each with a different frequency? Bohr studied the light coming from hot gases, or gases he'd passed electricity through, with one of these, a spectroscope, which is basically a box with a diffraction grating and a little ruler in there to measure the wavelength of light produced. And what he found was incredible. Each excited hot gas makes its own unique emission spectrum, individual bands of light of unique wavelengths and frequencies. It's like a signature for each element. Bohr repeated the experiment, this time shining bright white light through cold gases, gases from the electricity going through them, and this time he got the exact opposite effect, a full spectrum interrupted by small black breaks where the individual bands of light should have been. This is called an absorption spectrum. If these two spectra are superimposed, we see a continuous spectrum. So what about these atoms cause the absorption or emission of light, and why only light of particular wavelengths of frequencies? Well the answer came from a new idea that was sort of floating around at the time called quantum physics. Quantum physics is based on the idea that energy exists in little discrete packages. So Bohr hypothesized that atoms absorbed and emitted these little packages of energy in the form of light, or photons. So how does an atom do that? Well Bohr knew that the planetary model couldn't explain it, so he came up with a new model of the atom. Bohr's model had a nucleus and electron orbitals, or energy levels. These energy levels are fixed for a particular atom, they can't change or vary. And while the electron is in an energy level, it doesn't emit or absorb light. We say that an electron remaining in one energy level like this is in a stationary state. Electrons usually start off close to the nucleus, in the ground level or ground state, but they can move up or transition to the next energy level, called the first excited state, if they absorb just the right amount of energy from an incoming photon. If the electron absorbs the amount of energy needed to transition up two levels, it can get up to the second excited state. In fact, an electron can gain so much energy from a photon, it can leave the atom altogether, or ionize. But if the electron doesn't ionize, eventually the electrostatic force causes the electrons to be attracted back to the protons in the nucleus, so the electrons transition back down, and in doing so, release their energy as photons again. These transitions are what caused the emission and absorption lines Bohr observed initially. Each downwards transition, from level 3 to 2, or 3 to 1, or 2 to 1, emits a photon that shows up on the emission spectrum. And the upwards transitions, where the photons were absorbed and their energy used to move the electron up, shows up in the absorption spectrum. Bohr's model gave physicists a better understanding of what was happening inside of atoms, because it was able to explain more observations. But it wasn't perfect, it really worked well for hydrogen, but not as great for other elements. So in order to improve on this model, we need to bring in new physics. And you can take a look at this video, where we consider electrons as both waves and matter, in order to expand on the Bohr model, and come up with an even better model of the atom.