Models of the Atom
In the early 1900s, scientists around the world began to refine and revise our understanding of atomic structure and sub-atomic particles. Scientists understood that matter was made up of atoms, and J.J. Thompson had shown that atoms contained very small negative particles known as electrons, but beyond that, the atom remained a mystery.
New Zealand scientist Ernest Rutherford devised an experiment to better understand the rest of the atom. The experiment, known as Rutherford’s Gold Foil Experiment, involved shooting alpha particles (helium nuclei) at a very thin sheet of gold foil, and observing the deflection of the particles after passing through the gold foil. Rutherford found that although most of the particles went through undeflected, a significant number of alpha particles were deflected by large amounts. Using an analysis based around Coulomb’s Law and the conservation of momentum, Rutherford concluded that:
- Atoms have a small, massive, positive nucleus at the center.
- Electrons must orbit the nucleus.
- Most of the atom is made up of empty space.
Rutherford’s model was incomplete, though, in that it didn’t account for a number of effects predicted
by classical physics. Classical physics predicted that if the electron orbits the atom, it is constantly accelerating, and should therefore emit photons of EM radiation. Because the atom emits photons, it should be losing energy, therefore the orbit of the electron would quickly decay into the nucleus and the atom would be unstable. Further, elements were found to emit and absorb EM radiation only at specific frequencies, which did not correlate to Rutherford’s theory.
Following Rutherford’s discovery, Danish physicist Niels Bohr traveled to England to join Rutherford’s research group and refine Rutherford’s model of the atom. Instead of focusing on all atoms, Bohr confined his research to developing a model of the simple hydrogen atom. Bohr’s model made the following assumptions:
- Electrons don’t lose energy as they accelerate around the nucleus. Instead, energy is quantized… electrons can only exist at specific discrete energy levels.
- Each atom allows only a limited number of specific orbits at each energy level.
- To change energy levels, an electron must absorb or emit a photon of energy exactly equal to the difference between the electron’s initial and final energy levels:
Question: Calculate the energy of the emitted photon when an electron moves from an energy level of -1.51 eV to -13.6 eV.
Question: What is the emitted photon’s wavelength?
Bohr’s Model, therefore, was able to explain the first two limitations of Rutherford’s Model. Further, Bohr was able to use his model to predict the frequencies of photons emitted and absorbed by hydrogen, explaining Rutherford’s problem of emission and absorption spectra! For his work, Bohr was awarded the Nobel Prize in Physics in 1922.
"If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet." — Niels Bohr