Demonstration with Magnets Valuable prize for balancing suspended magnet between sets of attracting ma gnt!

Chemistry 125: Lecture 2 Sept 4, 2009 Force Laws, Lewis Structures, and Resonance For copyright notice see final page of this file Congratulations to Saybrook! (first to complete their directory on the Wiki)

Are There Atoms & Molecules? What Force Holds Atoms Together?

What Holds Atoms Together? Hooks Hormones? The Hands of the Deity? Springs? Bolts? Friction? Clips?

What Holds Atoms Together? GravityQuarks Kinetic Energy Quantum Forces Strange Attractors Magnetic Forces Electrical ForcesThe Strong Force Shared Electron Pairs Exchange of Virtual Particles Exchange of Photons The Weak Force Let’s Vote

Robert Boyle ( ) P  V = const Air Pump built by his servant Robt. Hooke 1661

1678

Hooke’s Law Hooke’s Force Law F = -k  x

Scale Potential Energy Force   x Hooke’s Law “Ut tensio sic vis”  x2 x2 extension force energy

Isaac Newton ( )

Force : Gravity Attraction at a Distance vs. Cartesian blocked repulsion Newton : Force  r -2 (How about mass?)

Isaac Newton ( )

Query 31 in Opticks (1717) Have not the small Particles of Bodies certain Powers, Virtues, or Forces by which they act at a distance, not only upon the Rays of Light for reflecting, refracting and inflecting them, but also upon one another for producing a great part of the Phaenomena of Nature? For it's well know that Bodies act one upon another by the Attractions of Gravity, Magnetism and Electricity; and these Instances shew the Tenor and Course of Nature, and make it not improbable but that there may be more attractive Powers than these. For Nature is very consonant and conformable to her self.

Query 31 How these Attractions may be perform'd, I do not here con- sider. What I call Attraction may be perform'd by impulse, or by some other means unknown to me. I use that Word here to signify only in general any Force by which Bodies tend towards one another, whatsoever be the Cause. For we must learn from the Phaenomena of Nature what Bodies attract one another, and what are the Laws and Properties of the attraction, before we enquire the Cause by which the Attraction is perform'd, The Attractions of Gravity, Magne- tism and Electricity, react to very sensible distances, and so have been observed by vulgar Eyes, and there may be others which reach to so small distances as hitherto escape obser- vation; and perhaps electrical Attraction may react to such small distances, even without being excited by Friction.

Query 31 The Parts of all homogeneal hard Bodies which fully touch one another, stick together very strongly. And for explaining how this may be, some have invented hooked Atoms, which is begging the Question; and others tell us that Bodies are glued together by rest, that is, by an occult Quality, or rather by nothing; and others that they stick together by conspiring Motions, that is, by relative rest amongst themselves. I had rather infer from their Cohesion, that their Particles attract one another by some Force, which in immediate Contact is exceeding strong, at small distances performs the chymical Operations above mention'd, and reaches not far from the Particles with any sensible Effect. Maybe F chymical  1/r >2 ?

Query 31 …the Attraction [between glass plates separated by a thin film of Oil of Oranges] may be proportionally greater, and continue to increase until the thickness do not exceed that of a single Particle of the Oil.

Query 31 There are therefore Agents in Nature able to make the Particles of Bodies stick together by very strong Attractions. And it is the business of experimental Philosophy to find them out. (This business will take us nearly five weeks)

F electrical = F gravity = mass 1  mass 2 r2r2 charge 1  charge 2 r2r2 Astronomy Kepler, Newton (?)(?)

Initially wanted to be a mathematician in charge of building Ft. Bourbon, Martinique, Meyzières Engineering School National Maritime Museum, Greenwich ~1751 “Charles Augustin continued to deny his mother’s desire that he study medicine and was therefore temporarily disowned. Without funds, he was forced to join his father in Montpellier.” (Gillmour. 1971, p. 5) ( ) Institut de France Royal Corps of Engineers Académie Royale des Sciences electrical torsion balance 1785 Mémoires de l’Académie Royale des Sciences, pp Charles Augustin Coulomb ( ) 1793 silver wire (~20  m thick ~1/4 of a hair) gilded pith ball needle deflection scale torsion pointer with deg. scale (0°) Pointer Twist (°) Net Deflection (°) charge pinhead by rubbing

Pointer Twist (°) Net Deflection (°) (1) Coulomb used these data to derive his law for repulsion of like charges: F  1/r 2 How certain could he be that the exponent for r is exactly 2, and not 2 +  ? That is, how large a  could be consistent with his data? (Modern experiments, relevant to the rest mass of the photon and to the dimensionality of space, show  < ) Hints: One approach would be to make a plot based on numbers derived from these data. You might want to consider experimental error and geometry. Detail on experiment and calculation is available in the translation of Coulomb’s paper on the course website. Two Problems (2) Two years later (1787) Coulomb extended this law for repulsion to include attraction between opposite charges. Explain why Coulomb would need to develop a new apparatus for this experiment. That is, why couldn’t he just use the same apparatus with different charges on the two gilded pith balls? Hint: Remember that the torsional force is approximately linear in the displacement. It might help to graph the Coulombic and torsional energies through a region that includes the point where they balance.

Binding Energies from Various Sources Magnetic   1  2 /r Log (Potential Energy) kcal/mol Gravitational  m 1 m 2 /r Coulombic  q 1 q 2 /r Chemical Bond (similar to 1 e Coulombic) (What of Kinetic Energy?) (216 kcal/mol) : Proton-Electron at 1.54Å ( kcal/mol) : Electron Spins at 1.54Å (5 x 10 7 kcal/mol) : Proton-Neutron in Deuterium Nucleus (3 x kcal/mol) : C atoms at 1.54Å (90 kcal/mol) : C-C at 1.54Å “Strong” Binding

Is there a Chemical Force Law?

How far can you Stretch a Chain of Atoms before it Snaps?

Demonstration with Magnets Valuable prize for balancing suspended magnet between sets of attracting ma gnt!

Force Laws & Molecular Structure Spring (ut tensio sic vis) Electrical Charges (gravity, etc.) Balanced minimumBalanced minimum ! F = -k  x  F  = k / (  x) 2 Potential Energy Single MinimumDouble Minimum x  x sum Slope = F nd Spring (weaker, opposing) 3 rd Stronger Body E = k/2 (  x) 2 E = -k/(2 |  x|)

(but not with ions or magnets) Thus with springs you might make a stable polyatomic molecule from point atoms. However, if bonds obeyed Hooke’s Law, they could never break.

Fixed Neighbor Mathematically convenient approximation for realistic bond energies (proposed 1929) Sum Morse Potential Second Fixed Neighbor

Morse Potential Snaps at Inflection Point (Change from direct to inverse force)

What ARE bonds?

Why do Elements Differ? 1861 Different # for different atoms: H(1), C(4), O(2), N(3) NH 3 and NH 4 Cl or 5? ) 19 th Century Experiments led to VALENCE numbers

Gertrude and Robert Robinson (1917) Might Latent Valence Loop explain trivalence of pentavalent N? What does the loop mean? “partial dissociation” Such slippery concepts “explain” so much that they convince you of nothing. “latent” valence loop Why/When ? Why/When ? reactionproduct Reaction Scheme Might Partial Dissociation explain amine/HCl reactivity? How Many?

Electron Discovered 1897

The Cubic Octet of G. N. Lewis ( ) as Harvard Undergraduate ~1894 as Harvard Instructor ~1902 © E. S. Lewis, by permission

Octet to "Explain" Periodicity & Electron Transfer (1902 teaching notes)

Octet Predicts Shared Pair Bonding ? shared edge shared face

Cubic Octet to Tetrahedral Octet to Tetrahedral Octet N N :: (G. N. Lewis 1916) Tetrahedral distribution of the bonds from C had already been known in organic chemistry for 40 years!

Good Theory should be Realistic & Simple In regard to Facts it should allow: Prediction Suggestion Explanation Classification & Remembering as as possible Postdiction: Realm of Lore

End of Lecture 2 Sept 4, 2009 Copyright © J. M. McBride Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0) Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol. Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0

From Number of Valence Electrons we would like to predict: Constitution (valence numbers for different atoms) Structure (distances & angles ) Energy Content Reactivity Charge Distribution

Lewis Explains Constitution “the nature and sequence of bonds” H B C N O F H N H H Why Octet? Why Pair for H / He? (Electron #  Valence # and Unshared Pairs)

H H H N + H H H B HCN H C Tetravalent N is positive. N C H N C H N Tetravalent B is negative. NH 3 BH 3 H 3 N-BH 3 + H H H N H H H B Bookkeeping of “Formal” Charges (each atom is assigned half-interest in bonding pairs) Puzzle: 2 BH 3  B 2 H 6 + ~40 kcal/mol What is the “glue”? (Answer in Lecture 16) Lewis had the idea of using : to denote unshared pairs.

+ - *) Energy of a proton on the “molecular surface” Surface Potential* of H 3 N-BH 3 (from Quantum-Mechanics) HIGH (+ 25 kcal/mole) (-41 kcal/mole) LOW N end indeed bears positive charge and B end bears negative charge

Lewis Explains “Pentavalent” N. Actually Tetravalent - thus Charged. N H H H H + Cl

Amine R R R N S R R Sulfide O Oxide O O oxide one O Peroxys

also for HCNO (CNO in all six linear orders, plus ring) Draw Lewis Dot Structures for: H N C (in the order shown) Start Lewis-Drill Problems:

Start Memorizing Functional Groups

Double Minimum equilibrium EQUILIBRIUM vs. RESONANCE O HCN + - HCN O + - all octets charge sepn  all octets still charge sepn  poorer site for -  N position Energy midway left shift : to eliminate charge sepn.shift : to restore octet N closer to C than to O N ~midway between C and O Geometric Implication? but maybe in truth…

EQUILIBRIUM vs. RESONANCE HC O N + - N position Energy midway left Single Minimum resonance O HC N + - single compromise position for N

Choice between Resonance and Equilibrium must be based on experimental facts (or a better theory) that can distinguish single from double minimum

Equilibrium vs. Resonance AB AB Two Real Species One Real Species Two “Reasonable” Structural Formulas Failure of Simplistic Notation Unusually Stable Compared to what?

Equilibrium vs. Resonance H C O O H HC O O H H C O O HC O O Two Species Two Species? H C O O HC O O One Nuclear Geometry! One Species! (Evidence: Infrared Spectroscopy) LORE (Evidence: Electron Paramagnetic Resonance)

What Holds Atoms Together? GravityQuarks Kinetic Energy Quantum Forces Strange Attractors Magnetic Forces Electrical ForcesThe Strong Force Shared Electron Pairs Exchange of Virtual Particles Exchange of Photons The Weak Force Let’s Vote 0 59 (unanimous)