|We are now in Chapter 12 of the textbook. Many homework problems, though, continue with drilling on aspects of stoichiometry.||
|Lecture outline.||Mass spectra (molecules)
Problems with some results of physical measurements
(Old Quantum Theory)
New Quantum Theory)
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
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)|