demonstration with magnets valuable prize for balancing suspended magnet between sets of attracting...
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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)
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
Force : GravityAttraction at a Distance
vs. Cartesian blocked repulsion
Newton : Force r-2 (How about mass?)
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)
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
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
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?
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
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.
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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
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:
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)
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