Lecture #3
We are now in Chapter 12 of the textbook. Many homework problems, though, continue with drilling on aspects of stoichiometry. CURMUDGEON GENERAL'S WARNING. These "slides" represent highlights from lecture and are neither complete nor meant to replace lecture. It is advised not to use these as a reliable means to replace missed lecture material. Do so at risk to healthy academic performance in 09-105.
Lecture outline. Mass spectra (molecules)

Problems with some results of physical measurements

Line spectra

Photoelectric Effect

Blackbody Radiation (and the ultraviolet catastrophe)

(Old Quantum Theory)

Physics puzzles "solved" by quantization

Blackbody radiation, Planck, E=hv, energy jumps

Photoelectric Effect, Einstein, the photon

Line spectra for hydrogen

New Quantum Theory)

deBroglie and wave-particle duality of matter

The mass spectrometer can also be used to determine molecular ion mass/charge ratios. Here is a schematic spectrum for ammonia, NH3. The largest peak (at m/q = 17, intensity arbitrarily set = 100) is that for NH3+ (mass = 14 + 1 + 1 + 1). The peak at m/q = 16, for example, is due to a fragment, NH2+. Mass specrometry is a very valuable technique. It is used extensively today for identifying molecules, even quite complex ones. Here is an example of the mass spectrum for a simple molecule, carbon dioxide, illustrating how structural information about the intact molecule can be obtained from the mass pattern of its break-up fragments. Three possible bonding arrangements are shown. In two, it would be expected that an O2+ ion would appear in the mass spectrum, but no such ion appears. It is reasonable to conclude that the correct arrangement is OCO. One of the major puzzles in experimental physical science near the end of the nineteenth century was the spectrum of light emitted by a "body" at different temperatures. Classical theories of physics were extremely successful in many arenas, but for radiation spectra predicted that intensity would vary inversely with (l)4.

(You recall what l represents, right?)

Max Planck will propose a startling solution, that energy takes place in discrete jumps...is quantized. For the vibrationsin solid matter, representing "heat", the energy is given by integral multiples of hn. Then the entire spectrum is explained for all wavelengths, for all temperatures.

E=hn where h is now known as Planck's constant and n is a vibration frequency. An explanation for the mysterious photoelectric was totally lacking at the end of the last century. Some colors of light could eject electrons from certain surfaces. And if the correct colors were chosen, no matter how dim the light, an electron could be ejected as soon as the light struck the surface. Albert Einstein uses Planck's idea about quantization to quantize the energy associated with light; that light exists in discrete packets whose individual energies are determined by their frequency. These photons are thus able to have high enough frequencies (short enough wavelengths) to eject photoelectrons. In contrast, they are also thus capable of having insufficient energy if their frequencies are too low. Conservation of energy relates the incident "photon" energy to the ejected photoelectron kinetic energy as highlighted here. Early attempts at explaining some serious puzzles in physics established what is now referred to as "old quantum theory". The hydrogen line spectra frequencies were consolidated by Rydberg into a single equation involving integers. Bohr's derivation of his planetary model involved an "arbitrary quantization" of angular momentum. Bohr's derivation led to prediction of discrete orbital radii for an electron moving about a nucleus and also for discrete energies that the electron was permitted to have. Illustrating the transition from shell 3 to shell 2 in Bohr's planetary model. The energy difference shows up as the energy associated with a quantum of electromagnetic radiation; that is, a photon The transitions between levels as observed in spectra and as predicted by Bohr's model for the hydrogen atom. Summary of the Bohr Planetary Model (for one electron; old quantum theory) 