Chem 125 Lecture 2 9/7/01 Projected material This material is for the exclusive use of Chem 125 students at Yale and may not be copied or distributed further. It is not readily understood without reference to notes from the lecture.

What Lewis Added

Text p. 16 “empirical rules for assessing the relative importance of the resonance structures of molecules and ions. 1. Resonance structures involve no change in the positions of nuclei; only electron distribution is involved. 3. The more important structures are those involving a minimum of charge separation, particularly among atoms of comparable electronegativity. Structures with negative charges assigned to electronegative atoms may also be important. 2. Structures in which all first-row atoms have filled octets are generally important; however, resulting formal charges and electronegativity differences can make appropriate nonoctet structures comparably important. LORE

Levitator by Martin Simon (UCLA) Eppur sta fermo

In systems governed by inverse-square force laws there can be no local minimum (or maximum) of potential energy. Earnshaw's Theorem (1839)

Visualizing Earnshaw - Coulomb's Electrostatics

A positive particle has a local maximum or minimum of energy only at the location of another charged particle, never in free space.

Force Laws

J.J. Thomson ( ) Electron (1897) Plum-Pudding Atom