Postulates of Bohr model Electrons can have only certain specific values of energy. Electrons travel in fixed, circular orbits* (* = we no longer believe this postulate)

Postulates of Bohr model Energy absorbed Energy Released as light When an electron absorbs the exact difference in energy between 2 levels it makes a quantum leap up When an electron makes a quantum leap down it releases the exact difference in energy between two levels.

Link to Electron Orbits

Foundations of Modern Quantum Mechanics Bohr Model In 1913, Niels Bohr introduced the concept of quantized energy levels for electrons. Modern quantum mechanics accepts quantized energy levels, but rejects Bohr’s circular orbits.

Failure of Bohr Model Accurately predicts only part of hydrogen spectrum. Fails to predict the spectra of any atom with more than 1 e-. Fatal Flaw: e- don’t move in fixed circular orbits

Heisenberg Uncertainty Principle It is impossible to accurately determine both the position and momentum of an electron at the same time. (Momentum = mass x velocity). Fill in blank: Short wavelength = high energy waves to locate e- Link to video Link Heisenberg explained

Trying to pinpoint the location of a tiny electron requires a collision with short wavelength high energy photon which then changes the speed and direction the electron is traveling in an unpredictable way.

Implication of Heisenberg Uncertainty Principle The implication of the Heisenberg Uncertainty Principle is that it is impossible to trace the exact trajectory (path) that an electron will follow.

Implication of Heisenberg Uncertainty Principle It is only possible to describe the probability of an electron being in a particular region of space at a particular time.

De Broglie Wave-Particle Duality Louis de Broglie originated the idea that all matter and energy displays properties of both particles (with mass) and waves (with wavelength). Link to Wave-Particle duality

Standing waves with wavelengths that “fit” in the space

Only certain specific wavelengths of light will fit in the space occupied by the standing wave without destructive interference. De Broglie standing wave video

Since wavelength determines energy, restricted wavelengths means restricted energies.

Results of Schroedinger’s Work Schroedinger described the behavior of each electron in an atom by using something called a “wave function”, symbolized by the Greek letter psi: While itself has no physical meaning, 2 corresponds to the probability of finding an electron in a particular region of space (orbital). The requirement of fitting a wave function in a particular region around the nucleus means that only certain wavelengths (and therefore energies) are possible for a given electron. Treating the electron as a standing wave explains why electron energy levels must be quantized! Link to Hydrogen Phet

Image of orbital (space in which there is a 90% chance of finding an electron)

Link to Double Slit/ Heisenberg Link to Brian Greene (use first 20 min) Link to Double slit quantum