Demonstrationwith

Magnets

Valuable prize for balancing suspended magnet

between sets of attracting ma gnt!

Chemistry 125: Lecture 2Sept 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 HoldsAtoms Together?

What Holds Atoms Together?Hooks

Hormones? The Handsof the Deity?

Springs? Bolts?

Friction?Clips?

What Holds Atoms Together?Gravity Quarks

Kinetic Energy

Quantum Forces

Strange Attractors

Magnetic Forces

Electrical Forces The Strong Force

Shared Electron Pairs Exchange of Virtual Particles

Exchange of Photons

The Weak Force

Let’s Vote

Robert Boyle

Robert Boyle(1627-1691)

P V = constAir Pump built by his servant Robt. Hooke

1661

1678

Hooke’s Law

Hooke’s Force LawF = -k x

Scale

Potential Energy

Force x

Hooke’s Law“Ut tensio

sic vis”

x2

extension

forc

een

ergy

Isaac Newton (1643-1727)

Force : GravityAttraction at a Distance

vs. Cartesian blocked repulsion

Newton : Force r-2 (How about mass?)

Isaac Newton (1643-1727)

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 31How 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 Fchymical 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)

Felectrical =

Fgravity =mass1 mass2

r2

charge1 charge2

r2

AstronomyKepler, Newton

(?)

Initially wanted to be a mathematician

in charge of building Ft. Bourbon, Martinique, 1764-1772

Meyzières Engineering School1760-1761

Nat

iona

l Mar

itim

e M

useu

m,

Gre

enw

ich

~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)

(1732-1799)

Institut de France1795-1806

Royal Corps of Engineers1760-1790

Académie Royale des Sciences1781-1793

electricaltorsion balance

1785Mémoires de l’Académie

Royale des Sciences,pp. 569-577

Charles Augustin Coulomb(1736-1806)

1793silver wire

(~20 m thick ~1/4 of a hair)

gilded pith ball

gilded pith ball

needledeflection

scale

torsion pointerwith deg. scale

(0°)P

oin

ter

Tw

ist

(°)

Net

Def

lect

ion

(°)

0 36

126 18

567 8.5

charge pinhead by rubbing

Poi

nte

r T

wis

t (°

)

Net

Def

lect

ion

(°)

0 36

126 18

567 8.5

(1) Coulomb used these data to derive his law for repulsion of like charges:

F 1/r2

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 < 10-17)

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 12/r3

0 3 6-3Log (Potential Energy) kcal/mol

Gravitational m1m2/r

Coulombic q1q2/r

Chemical Bond

(similar to 1 e Coulombic)

(What of Kinetic Energy?)

(216 kcal/mol): Proton-Electron at 1.54Å

(0.0014 kcal/mol): Electron Spins at 1.54Å

(5 x 107 kcal/mol): Proton-Neutron in Deuterium Nucleus

(3 x 10-32 kcal/mol): C atoms at 1.54Å

(90 kcal/mol): C-C at 1.54Å

“Strong” Binding

Is therea ChemicalForce Law?

How far can you Stretcha Chain of Atomsbefore it Snaps?

Demonstrationwith

Magnets

Valuable prize for balancing suspended magnet

between sets of attracting ma gnt!

Force Laws & Molecular StructureSpring (ut tensio sic vis) Electrical Charges (gravity, etc.)

Balanced minimum Balanced minimum !

F = -k x F = k / (x)2

Pot

entia

l Ene

rgy

Single Minimum Double Minimum

x

x

sum

sum

Slope = F

0

0

2nd Spring(weaker,

opposing)

3rd 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.

FixedNeighbor

Mathematically convenient approximation for realistic bond energies

(proposed 1929)

Sum

Morse Potential

Second Fixed

Neighbor

Morse Potential

Snaps atInflection 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)

NH3 and NH4Cl

or 5?)

19th 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 ? reaction product

Reaction Scheme

Might Partial Dissociation explain amine/HCl reactivity?

How Many?

ElectronDiscovered

1897

The Cubic Octet of G. N. Lewis

(1875-1946)

as Harvard Undergraduate~1894

as Harvard Instructor~1902 ©

E. S

. Lew

is, b

y pe

rmis

sion

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

Octet Predicts Shared Pair Bonding

?

shared edge

shared face

Cubic Octet to Tetrahedral Octetto 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 Factsit should allow:

PredictionSuggestion

Explanation

Classification& Remembering

as as possible

Postdiction:Realm of Lore

End of Lecture 2Sept 4, 2009

Copyright © J. M. McBride 2009. 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).

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 Electronswe 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••

••••

•

1 233 4 1

H•N••

••••

•HH

Why Octet?

Why Pair for H / He?

(Electron # Valence # and Unshared Pairs)

H •

H•

H•N•

••

••+

H•H•

H•B•

••

HCNH • •

•C ••

Tetravalent N is positive.

N ••

••

•

••CH•

• ••• N••• C ••H N

Tetravalent B is negative.

NH3 BH3H3N-BH3

+

H •

H•

H•N ••

••

•H•

H•

H•B•

••

Bookkeeping of “Formal” Charges

(each atom is assignedhalf-interest in bonding pairs)

••

Puzzle:

2 BH3 B2H6 + ~40 kcal/molWhat 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 H3N-BH3 (from Quantum-Mechanics)

HIGH(+ 25 kcal/mole)

(-41 kcal/mole)

LOWN end indeed bearspositive charge

and B end bearsnegative charge

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

NH

HHH

+Cl

Amine

R •

R•

R•N ••

••

•

SR •

R••

••

Sulfide

•

••

O ••

• •

• •Oxide

O• •

••

••

O ••

• •

• •oxide

+ -

+ -+2 --

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

DoubleMinimum

equilibrium

EQUILIBRIUM vs. RESONANCE

OH C N••

••• • +-

H C N O••

••

••

+ -all octets charge sepn

all octets still charge sepn poorer site for -

N position

Ene

rgy

midwayleft

shift : to eliminate charge sepn.shift : to restore octet

N closer to Cthan to O

N ~midwaybetween C and O

Geometric Implication?

but maybe in truth…

EQUILIBRIUM vs. RESONANCE

H C O••

••

••N+ -

N position

Ene

rgy

midwayleft

SingleMinimum

resonance

OH C••

••• •N+-

single compromise

position for N

Choice between

Resonance and Equilibrium

must be based onexperimental facts

(or a better theory)

that can distinguish singlefrom double minimum

Equilibrium vs. Resonance

A B A B

Two Real SpeciesOne Real Species

Two “Reasonable”Structural Formulas

Failure of Simplistic NotationUnusually StableCompared to what?

Equilibrium vs. Resonance

H CO

O

HH C

O

O H••

••••

•• ••

••

••

••

••

H CO

O

H C

O

O

••••

•• ••

••

••

•••

•

Two Species

Two Species?

••

H C

O

O

H C

O

O

••••

•• ••

••

••

•••

•

One Nuclear Geometry!

•

•One Species!

(Evidence: Infrared Spectroscopy)

LORE

(Evidence: Electron Paramagnetic Resonance)

What Holds Atoms Together?Gravity Quarks

Kinetic Energy

Quantum Forces

Strange Attractors

Magnetic Forces

Electrical Forces The Strong Force

Shared Electron Pairs Exchange of Virtual Particles

Exchange of Photons

The Weak Force

Let’s Vote

0

59 (unanimous)

10

5

20

58

0

7

12

3

5

5