Lecture #2
  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 History (Stoichiometry and Periodicity)

Early recognition of "Periodicity"


Kinetic Energy

Potential Energy

Internal Energy


Wavelength, frequency, amplitude, wave number


H-atom line spectra (emission and absorbtion)


Mass spectra

Avogadro accepts the significance of Gay-Lussac's experiments and hypothesizes some significant ideas.
Dalton disputes the significance of Gay-Lussac's results and regards Avogadro's ideas as erroneous.
Nearly fifty years go by before order is restored by Cannizzaro to the "science" of chemistry.
Simple measurements on equal volumes of gases in conjunction with chemical analysis of the composition of the gases and acceptance of Avogadro's hypotheses leads to a unique collection of "combining weights" of the atoms.
Gas analysis data on oxygen-containing gases. Note that there are three compounds on this list that contain only nitrogen and oxygen. The data can be used to demonstrate the Law of Multiple Proportions.
Mendeleev recognizes the periodic behavior of combining ratios (or "valency") of the atoms of elements.
(This was missing from lecture.) In 1869, the Russian chemist Mendeleev noted that the repeating patterns of behavior could be arranged in a sequence of elements giving rise to the "Periodic Table" of the elements.  
We'll be spending some time looking at light and the energy associated with light. First, let's establish some terms related to energy and couple of familiar examples of potential energy. Potential energy is related to forces acting on a particle.  
 The different types of energy that we will distinguish in our upcoming discussions.
By the end of the 1800s, the nature of light was moderately well understood. Light has an oscillating electric and magnetic field. The amplitude of a "light wave" is shown
The wavelength of a light wave
Wavelength, frequency and wave number are related through the speed of the traveling wave. (Wave number is not discussed in the text. It will show up in other courses and is presented here for completeness. You need not know it for 09-105.)
Dispersion of white light through a prism producing the familiar rainbow spectrum. You need to know the wavelengths of blue and red light.
A serious puzzle involving light was the existence of line spectra, one example of which is shown here: the line spectrum of the hydrogen atom. Emissions and absorbtions of light (energy) occur at very precise wavelengths (energies).Why? The answer was not known yet.
The first fundamental particle is discovered, the electron. Structure of the atom is about to become a bit clearer.
Thompson's student Rutherford determines the structure of an atom by studying the scattering of fast charged particles from atoms. He finds most of the mass and all the positive charge concentrated in a small central core, called the nucleus. The number of units of positive charge, Z, will determine the element (in contrast to Dalton's postulate declaring it was weight/mass that determined the element). The number of electrons exactly balances the number of positive charges for an atom, which, by definition is neutral...uncharged.
A valuable contribution is made by the invention of the mass spectrometer in which particles are separated according to their mass-to-charge ratio, m/q. (H is the strength of the magnet.) This is like Figure 2-19 in the text.  
Isotopes are revealed in mass measurements. They are due to the second constituent of nuclei, the neutrons, which are similar in mass to protons but which have no electrostatic charge. Isotopes are indicated by mass numbers -- integers -- which are placed as superscripts by the symbol, usually in front of the symbol but sometimes after it.
The existence of isotopes is confirmed in which distinct atomic masses of the same element (neon in this case, as in Figure 2-20) are separated by a mass spectrometer.  
An absolute mass scale is needed to translate our relative atomic masses into something that can actually be measured without having to use hydrogen gas, for example. The benchmark has changed over the years and has currently settled on using the isotope C-12 as the link. The term "mole" enters our vocabulary and is an extremely useful shorthand word representing a very large number...Avogadro's number.
In fewer words, mass of a substance and number of moles of a substances are two different ways of talking about amounts. Stoichiometry involves amounts.