CH301H - Principles of Chemistry I: Honors
Fall 2017, Unique 50135

Lecture Summary, 5 September 2017


Introduction: Today we discussed basic problem solving techniques.  We covered the importance of careful analysis of units and the magnitude of the result.  Whenever you solve a problem, you should look at your final answer and ask yourself qualitiative questions about whether that answer makes any sense.  Do the units make sense?  Should the number be > or < 0?  Should it be > or < 1?  Does the magnitude of the number make sense?  If your number doesn't make good physical sense, then you have made a mistake that you need to go back and find.  Getting into this habit will allow you to solve complex problems and have confidence in your answer, even if you don't know what it is.

Potential Energy Diagrams:  To begin studying the classical model of chemical bonding, we thought about electrostatic forces contributing to the structure and stability of the atom.  We reviewed Coulomb's law to calculate electrostatic force (F) and electrostatic potential energy.  We used a calculation of potential energy (V) to construct a potential energy diagram (V(r) vs. r) for several examples of interacting charges (proton + electron, proton + proton, etc.).  We found that when F < 0 and V < 0, the system is in an attractive or stabilized state, and when F > 0 and V > 0, the system is in a repulsive or destabilized state.  We then used this information to reconstruct Rutherford's gold foil experiment, and used his data to estimate the size of the Au nucleus. We found that under the correct experimental conditions, we could calculate a very accurate estimate of the Au nuclei radius, although our estimate must necessarily be slightly higher than the actual value.