- Principles of Chemistry I: Honors
Fall 2016, Unique 50015
Lecture Summary, 20 September 2016
Model of the Atom: In 1913, Bohr attempted to
define a model to explain the stable atom. He started with
two assumptions: 1) an electron orbits around a positively charged
nucleus according to Rutherford's planetary model, and 2) the
angular momentum of the orbiting electron is quantized. From
these two postulates, he derived an expression for the energy of a
one-electron atom based only on constants and the "quantum number"
n. The derivation
of this expression is given here:
The Bohr model was able to explain the ionization energies and observed emission spectra of one-electron atoms.
Atomic Spectroscopy: The electromagnetic spectrum can be resolved into components of vastly different frequency and wavelength. Although the visible spectrum covers only a tiny fraction of the electromagnetic spectrum(about 2.5 eV), because our eyes are sensitive to only this spectrum, we spend a lot of time thinking about how matter interacts with light at these wavelengths. By discharing an electrode into a pure gas, collisions between the free electrons and the molecules of gas will provide energy to move electrons in the gas to higher energy levels. When that electron falls back (or "relaxes") to its lowest energy(or "ground") state, it will emit energy in the form of light. We looked at a few examples of noble gasses that emit light in the visible region of the spectrum, and saw that sometimes the gas will emit several different wavelengths of light. You can perform this experiment on your own using a PhET Java applet:
By the end of the 19th century, these spectra lines had been measured for many atoms, and it was known empirically that the energy (i.e. frequency) of light emitted in these experiments was quantized.