A Quick History of
Chemistry
With thanks to Isaac Asimov
2,400 years in 42 slides!
S
As easy as LMN
S No one knows where the Latin word “elementum”
comes from. We get our word ELEMENT from it.
S Some think maybe the Romans had an expression that
something was as simple as “L-M-N,” just as we say
something is as easy as “A-B-C.”
S We use the word element to refer to a substance which
cannot be broken down into a simpler substance.
Democritus—400 BC
(http://www.gap-system.org/~history/Mathematicians/Democritus.html)
The Alchemists: 1400-1600?
(http://www.crystalinks.com/bacon.html)
Roger Bacon was an English alchemist
may have been the first European
to invent gunpowder.
Alchemists actually discovered some
chemistry, but mostly they were trying
to figure out a way to turn lead into
gold.
An Alchemist at work…
(http://en.wikipedia.org/wiki/Alchemy)
Paracelsus was a famous alchemist who may
be depicted in this famous painting.
He discovered many of the properties of salt
in the 1500s.
Salt had been known for thousands of years,
and in ancient times was so valuable, it was
actually used as money.
He was one of the first realize that elements
could come together to form compounds,
with different properties from from the
elements they are made from.
Robert Boyle…the Last of the
Alchemists
(http://en.wikipedia.org/wiki/Robert_Boyle)
More on Boyle
S Boyle started calling himself a “chymist” because the term alchemist
had gotten a bad reputation. The spelling was eventually changed to
chemist.
S He questioned the old Greek notions of the elements. He didn’t
believe water was an element, and he experimented on water to try
and prove that. Even though he didn’t quite prove that, he believed
water to be a compound, and the move away from Aristotle’s theory
of the elements had begun.
S He also experimented with cold temperatures, and whether water
froze at different rates based on the initial starting temperature. He
also scientifically explained some of the properties of ice.
Elements
S By the early 1600s, a few substances were known, but they weren’t known to
be elements yet. Gold and silver and copper and lead had been used since
the ancient times, but no one knew they were elements.
S The alchemists actually did discover 4 elements in the middle ages (As, Sb,
Bi and Zn).
S By 1700, about 14 elements were known. By the end of the 1700’s, around
1783, another 11 were known.
S Chemistry was evolving during this time, but few chemists paid attention to
the quantitative aspects of chemistry. They observed, but they didn’t
measure.
Phlogiston and Priestly
(http://en.wikipedia.org/wiki/Joseph_Priestley)
Antione Lavoisier
(http://www.answers.com/topic/antoine-lavoisier)
Father of Modern Chemistry
Proved that air was composed of 1/5 oxygen
and 4/5 nitrogen
Demonstrated experimentally the principle later
renamed “The Law of Conservation of Mass.”
Proved that hydrogen and oxygen combine to
form water, proving at last that water was a
compound.
Beheaded on 5/2/1794 by guillotine during the
French Revolution at age of 50.
More on Lavoisier
S By insisting on careful measurement and thoughtful
experimentation, Lavoisier turned chemistry from a
series of interesting observations into a real science.
S He explained the results that others had gotten. They
knew what they had done. Lavoisier helped to explain
why these things had happened.
S He studied combustion reactions and discovered the
importance of oxygen in both combustion and
respiration (disproving phlogiston in the process).
More on Lavoisier
S He also invented the system of naming
chemicals that we use today.
S Prior to Lavoisier, people who discovered
things named them whatever they wished.
S He also published the first modern chemistry
text (Traité élémentaire de chimie) thus
spreading his knowledge literally around the
world.
John Dalton
(http://www.intute.ac.uk/sciences/blog/wp-content/uploads/2007/09/johndalton.jpg)
A Quaker schoolmaster (became a teacher at the
age of 12) who studied all sciences, but made his
greatest contributions in chemistry.
Developed Atomic Theory and Law of Multiple
Proportions.
Atomic Theory helped to explain many of the
observations that scientists were making.
Law of Multiple Proportions helped to explain
that 2 elements could combine to form more than
1 compound; for example CO and CO2.
Dalton’s Atomic Theory
S 1. All elements are composed of tiny indivisible particles called
atoms.
S 2. Atoms of the same element are identical. The atoms of any one
element are different from those of other elements.
S 3. Atoms of different elements can chemically combine with one
another in small whole-number ratios to form compounds.
S 4. Chemical reactions occur when atoms are separated, joined or
rearranged. Atoms of one element cannot be changed into atoms of
another element by chemical rxns.
Indivisible?
S Well, Dalton did this work in the early 1800’s.
S We know now that atoms are composed of protons, neutrons and
electrons. Dalton didn’t know about them—they hadn’t been
discovered yet!
S HOWEVER, the atom is “the smallest part of an element that retains
the properties of that element.”
S So an atom of gold is still gold and is different from an atom of
carbon.
S Dalton’s model of the atom is called the “solid sphere” model.
Dmitri Mendeleev
S Mendeleev organized the Periodic Table by
atomic mass.
S He left “holes” in his table for undiscovered
elements and challenged the scientific world to
“find them!”
S In the early 20th century, Englishman Henry
Moseley reorganized the Periodic Table by
putting it in order of atomic number.
S Element 101 Md (Mendeleevium) is named after
him. Moseley has not been so honored yet.
JJ Thompson Discovered the
Electron in 1897.
(http://www.manep.ch/img/photo/challenges/nanotubes/thompson.jpg)
Electrons are negatively
charged and have almost
no mass at all, compared
to a proton.
Thompson revised
Dalton’s model of the
atom with one of his
own, called the “Plum
Pudding Model.”
Plum Pudding Model
(http://en.wikipedia.org/wiki/Plum_pudding_model)
Plum Pudding is a British dessert in which
plums are scattered more or less randomly
throughout a cake (the pudding).
Thompson knew atoms contained
electrons, and knew they were negative.
He also knew that the atoms overall were
neutral.
So, he proposed that the negative electrons
were randomly distributed throughout. The
rest of the atom was positively charged.
Thompson proposed the electrons were
moving in a circular fashion within the
positively charged “rest of the atom.”
Robert Millikan and the
Oil Drop Experiment
http://educar.sc.usp.br/licenciatura/2003/mi/Millikan-Oil-Drop-Apparatus.gif
S Electrons are negatively charged. Each electron has a charge of -1. (Don’t
forget the negative sign…it’s VERY important!)
Ernest Rutherford’s Nuclear
Model…it’s now 1910 or so…
(https://reich-chemistry.wikispaces.com/file/view/Ernest_Rutherford.JPG)
The Plum Pudding Model wouldn’t last long,
because one of JJ’s former students did some
experiments that forced the model to be
revised again. Like his mentor, JJ Thompson,
Rutherford won the Nobel Prize for his work
His “work” was the famous “gold foil”
experiments, where he was researching alpha
particles (see Chapter 28 stuff again).
As sometimes happened, Rutherford didn’t
set out to discover what he actually did.
Rutherford is also credited with discovering
the proton around 1919.
The Gold Foil Experiment
(http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf)
Check out the link!
(http://www.rsc.org/chemsoc/timeline//pages/1911.html)
Reference for below…
Rutherford created a device
to “shoot” α particles at a thin
piece of gold foil, literally only
a few atoms thick.
He expected them to go
through with little or no
deflection.
But that’s NOT what
happened. Some bounced
straight back as if they had
hit a brick wall!
Shocked, SHOCKED!
S Rutherford was completely surprised by this
result. He had accidentally discovered the
nucleus.
S Rutherford figured out that most of the mass
of the atom was contained in a small, dense
center which was positively charged.
S The electrons still rotated around the nucleus,
but most of the atom was composed of
“empty space.”
Neils Bohr: The Planetary Model
& Energy Levels
(http://www.usd.edu/phys/courses/phys300/gallery/clark/bohr.html)
Rutherford’s nuclear model only really lasted
for about 3 years, before Neils Bohr revised it
again.
Bohr asked a question: if the electrons are
rotating around the nucleus, why don’t they
“run out of energy.” As they did, they would
come closer and closer, attracted by the
opposite charge of the nucleus, and eventually
collapse onto the nucleus, destroying the atom
in the process.
This doesn’t happen, and Bohr answered why.
Bohr’s Planetary Model
(http://www.rsc.org/chemsoc/timeline//pages/1913.html)
An old
Bohr?
The electrons don’t just orbit anywhere.
They can only move in orbits that Bohr called
“energy levels.” Each energy level has a certain
amount of energy.
Electrons can move to a higher energy level by
gaining (or absorbing) energy. Or they can drop to
a lower energy level by losing (or emitting) energy.
But they can’t “run out” of energy, because in order
to stay in an energy level, they must have that
certain amount of energy.
Energy Levels
http://library.thinkquest.org/C006669/media/Chem/img/bohr.gif
S An energy level is a “region
around the nucleus where an
electron is likely to be
moving.”
S The first energy level (n = 1)
has the lowest energy. It is
called “the ground state.”
S Things in nature prefer to be
in the lowest possible energy
state.
Spectral Lines for H
http://www.daviddarling.info/images/hydrogen_spectrum.gif
S Electrons can ABSORB
energy and move to a higher
energy level.
S This is called “an excited
state.”
S When an electron loses
energy, it drops to a lower
energy level.
S We say it EMITS energy. If
The lines are characteristic for hydrogen.
They are like a fingerprint to identify H.
The Ballmer series is the only ones you can
see, but the others can be detected.
that energy is in the visible
part of the spectrum, we can
see those transitions.
More Bohr
http://www.daviddarling.info/images/hydrogen_spectrum.gif
S In Bohr’s model, the
energy levels get closer
together as you get further
away from the nucleus.
S If the electron gets far
enough away from the
nucleus, it can escape (n =
∞).
The electrons can jump from one level to
another. They can jump more than one level
at a time by absorbing or emitting enough
energy. An electron cannot jump to a spot
midway between levels (n ≠ 2.5)
S We no longer have an
atom. We have an ion,
since the atom has lost an
electron. The ion has a
positive charge.
Need for a Better Model
S Bohr’s model has some limitations.
S It worked very well for hydrogen (the simplest atom
with only 1 electron). It allowed scientists to make
detailed calculations that explains the behavior of H.
S It didn’t work for other elements, mostly because the
calculations were so detailed and complex they
couldn’t be done (the math hasn’t been invented yet).
S It also violated the Heisenberg Uncertainty Principle.
But no one knew that yet! We’ll get to that.
Heisenberg Uncertainty Principle
http://www.ostheimer.at/mambo/images/stories/Werner_Heisenberg_Tafel.jpg
S The Heisenberg
Since momentum = mass x velocity
and since the mass of the electron is
known, for all practical purposes, the
Heisenberg Uncertainty Principle says
that you can’t know both the position
of the electron and the speed of the
electron, at the same time.
Uncertainty Principle
states that for a very
small particle, such as
an electron, you
cannot know both its
exact momentum and
its exact position at
the same time.
So why does Bohr’s model violate
Heisenberg’s Principle
The Modern Model of the
Atom
S Many scientists (Louis DeBroglie, Max Planck, Albert
Einstein, Erwin Schroedinger, and many others) worked
on the model of the atom.
S Actually, they weren’t working on the model of the atom.
They were just working on cool and interesting scientific
problems. But they all made contributions to our current
understanding of the atom.
S Quantum mechanics is the “modern” model of the atom.
By the early 1930s, it had been “born.” It’s the model we
still use today.
Gee, bet this
guy never
amounted to
anything
Photoelectric Effect
http://www.guidetothecosmos.com/images/slide12_plus.jpg
S The photoelectric effect was
discovered by Albert
Einstein.
S He found that light of a
certain energy could
“knock electrons loose”
from certain metals.
S Einstein published “Theory
of Relativity” in 1906, long
before he worked on the
photoelectric effect.
Wait! What?
Light Knocks Electrons Off of
Atoms
S Alkali metals seem to be very prone to
this, if the light is of a sufficient energy.
S Einstein called this the photoelectric
effect. In this way, light is behaving not
as a wave but as a particle.
Photoelectric Schmotoelectric!
So What?
S Anyway, you might not be terribly impressed
with Einstein’s discovery.
S But, if electrons can be pried loose from the
metal, they can move around.
S If they can move around, the movement of
those electrons can generate a small amount
of electricity.
S If you can capture this electricity, you can do
useful work.
Like…Solar Power
S Solar power is based off of this principle. A
photoelectric cell is constructed which has a certain
type of metal in it.
S When sunlight shines on it, some of the electrons are
pried loose.
S The cell generates an amount of electricity.
S With hundreds or thousands of these in series, you can
take a small amount of power generated in each cell, and
multiply that by the total number of cells, and use that
generated power to do work in your house.
OK, well so what?
S This was one of the major assumptions
that helped lead scientists to quantum
mechanics.
S While in graduate school in France, a
young scientist named Louis de Broglie
asked himself this question
S If light can act as a particle, can a moving
particle also act as a wave?
De Broglie Equation
λ=h/mxv
http://jkphysics.in/images/De-broglie.jpg
S The answer was yes. DeBroglie found that
particles could act just like waves too!
S λ=h/mxv
S λ = wavelength, h = Planck’s constant, m =
mass, v = velocity
S The wavelength for a baseball pitched at 90
miles per hour, calculated using de Broglie’s
equation is 8.2 x 10-38 meter.
S We have no measuring instrument capable of
detecting such an incredibly small distance.
S But it was the next step in figuring out how the
atom was behaving.
The Final Pieces of the Puzzle
S Electrons have masses which are much, much less than a baseball,
and their wavelengths can be measured much more easily.
S So if particles could act as a wave, and electrons are particles,
would it help our understanding of the atom to think of
electrons as “waves?”
S The answer was yes and quantum mechanics was the result.
S Previously, scientists had treated electrons just as particles, and
tried to use all the normal math techniques that they used on
particles they could see. Those techniques worked well with large
particles, but with electrons, not so much.
Quantum Mechanics
http://www.hmi.de/bereiche/SF/SF7/PANS/english/nobel/Schroedinger/Schroedinger_01.jpg
S When Erwin Schroedinger
recalculated everything using the
“wave math” everything started to
come together and make total sense.
S He called this quantum mechanics.
S Later in life, he actually said this about
quantum mechanics…
S “I do not like it and I regret having had
anything to do with it.”
His simple-looking
but really complex
math equation.
S To which I add, ditto!
Still the best model we have
(and it’s from the 1930s)!
Quantum mechanics
has been around for
over 80 years now.
It still predicts the
behavior of atoms well,
and we haven’t found
anything better.
If anyone ever finds
anything better, I’ll let
you know
I met him in 1981!
http://sunsite.berkeley.edu/CalHistory/photos-large/seaborg.big.jpg
Dr. Glenn T. Seaborg
S That was 2,400 years of
history in one lesson!
S There’s lots more to
know and explore.
S Yes, you need to know
the important people and
what they did. It could
be on the SOL!
The End
Next: Chapter 4 powerpoint…
S