Chemistry 125: Lecture 2

Force Laws, Lewis Structures, and Resonance Newton suggested searching for a special force law to describe very short range attraction.

Neither direct (Hooke’s Law) nor inverse (Coulomb, Gravity) dependence on distance will do

– a composite like the Morse potential is needed. G. N. Lewis devised a “cubic-octet” theory

based on the newly discovered electron, and developed it into a shared pair model to explain

bonding. Lewis-dot notation and formal charges can be useful, but in some “single-

minimum” cases the Lewis formalism is inadequate and salvaging it required introducing the

confusing concept of “resonance.”

Synchronize when the speaker finishes saying

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Are There Atoms & Molecules?

What Force HoldsAtoms Together?

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 not F 1/r2 ?

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)

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?

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|)

Demonstrationwith

Magnets

(but not with ions or magnets)

Thus with springs you might make a stable polyatomic

molecule from point atoms.

However Hooke’s Law bonds would 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?

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

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?

(Electrons, 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)

+ -

*) Energy of a proton on the “molecular surface”

Surface Potential* of H3N-BH3 (from Quantum-Mechanics)

HIGH(+ 25 kcal/mole)

(-41 kcal/mole)

LOW

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

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)

End of Lecture 2Sept 5, 2008

Except for cited third-party material Chem 125 Lectures by J. M. McBride are licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Cited third-party material may be subject to additional intellectual property notices, information or restrictions. Users are responsible for checking the credits within the lectures and course material to identify third-party material, as such material may pose additional restrictions..