Download Document

It’s time for…
THE HISTORY OF THE ATOM
Aristotle- ~500 B.C.
philosopher
• What was the world made of?
– Water and Earth because it was everywhere
– Air because it was around and over us
– Fire because it was an agent of change
Democritus
500
B.C.
• Philosopher
• Proposed that matter is
made of about 100 basic
kinds of tiny, indivisible
particles he called
“atomos.”
• Nobody believed him.
His atoms would look like
this:
Who won?
• Aristotle’s theories
could be easily
understood
• The four elements could
be easily seen and
understood
• Democritus had no
proof
Smoke =air
fire
ashes = earth
400 B.C. - 1500 A.D.
The Dark Ages of Chemistry
Little research or
experimentation
done with atoms
(or anything else.)
John Dalton - 1803
Experimental Observations studying gases.
• CO vs CO2
• Ran electricity
through water
and…it decomposed!
• 2 H2O --> 2 H2 + O2
John Dalton
• All matter is made of indivisible particles called
atoms.
• All atoms of a given element are identical in mass &
properties.
• Atoms are not created or destroyed - just
rearranged in reactions.
• Different atoms can combine in simple ratios to
make compounds.
Atoms,
according to
Dalton:
Atoms are all
solid with
uniform density
(like a gum ball)
Cathode ray tube- running electricity through a
gaseous element
*beam of light” in a
was really made of
negative particles!
Discovered the electron!
Realized that electrons were smaller than
atoms.
This information discredited part
of Dalton’s atomic theory -Atoms were divisible after all.
The atom is made of smaller
particles (subatomic particles.)
Thomson’s
Model of
the Atom:
Plum
(electrons)
Pudding
(positive
stuff)
Today, it would probably be called the
“Chocolate Chip Cookie” Model
Ernest Rutherford
1911
• Performed the famous
gold foil experiment
• Hypothosis:
– If Thomson was correct
and atoms contained only
diffuse positive charge
then…
– Most of these heavy
positive particles should
go right through, right?
Experiment
Positive particles
Uranium
Gold foil
Detection screen
Rutherford shoots alpha
particles at a thin sheet of
gold foil. He wants to see
how tightly the protons and
electrons are packed.
He used gold because it can
be stretched thinnest of any
metal – only a few hundred
atoms thick!
He expects the alpha
particles to be deflected a
little by the gold atoms in
the foil.
He expects the particles to
be deflected a LITTLE if the
particles are loosely packed,
or a LOT if they are packed
tightly.
This is not what happens.
Instead, 99% of the alpha
particles go right through
the gold as if nothing was
there.
This meant that the atom
was mostly empty space.
Alpha particles, like
bullets, were going
straight through the gold
foil.
But once in a while, an
alpha particle would NOT
go straight through the
gold.
It would deflect and
bounce off at an angle.
This meant that there was
something small but solid
inside the atom.
The vast majority of alpha
particles would go straight
through, but . . .
. . .but occasionally one
would bounce off,
sometimes even straight
back!
1% are
deflected.
99% go
straight
through.
1% are
deflected.
Rutherford realized that
the only way both these
things could happen was
if all the heavy parts of
the atom . . .
99% go
straight
through.
1% are
deflected.
. . . were all concentrated
in a small spot in the
middle of the atom which
he named the nucleus . . .
99% go
straight
through.
1% are
deflected.
. . . while all the lightweight electrons circled far
away, leaving most of the
atom completely empty.
99% go
straight
through.
1% are
deflected.
Rutherford calculated
that the nucleus was so
small compared to the
whole atom . . .
99% go
straight
through.
1% are
deflected.
. . . that it was like a pea
in a football stadium.
But that pea would
contain ALL the weight
of the entire stadium.
99% go
straight
through.
Dense, positively charged nucleus.
Electrons surround the nucleus like bees
around a hive.
Atoms are mostly empty space.
Problem: Why didn’t electrons get
pulled into the nucleus?
Niels Bohr - 1913
• Used the Atomic Emission
Spectrum to explain electrons in
atoms.
What a handsome fellow!
Atomic Line Spectra
Expected to see a nice
progression of color just like
normal BUT…..
Gas such as
Neon or
Hydrogen
Prism or
diffraction
grating
Atomic Line Spectra
Instead he got just a few discrete
lines of color!
Gas such as
Neon or
Hydrogen
Prism or
diffraction
grating
For Electrons
31
21
Nucleus
32
Conclusions:
•Electrons can only exist in certain orbits at a specific distance
from the nucleus.
•Electrons could “jump” to higher orbits (absorbing energy).
When they “fell” back down (released energy), they would give
off light!
•If an electron stayed in its orbit, it didn’t lose any energy and
would not fall into the nucleus.
Bohr’s Atomic Model:
• Compares electrons orbiting
the nucleus of an atom to
planets orbiting around the sun.
• Electrons occupy distinct
orbitals.
• Orbitals have certain energies
and distances from the nucleus.
• Bohr’s theory explains how
light is given off – a huge
success!
Erwin Schrodinger
1926
• Used mathematical equations to study
electrons.
• Equation gives regions around the nucleus
where electrons are likely to be found.
• Electrons are located in regions called
electron clouds. These regions tell us where
electrons are most likely to be.
Schrodinger’s Model of the Atom:
2 Parts of the Atom…..
• Nucleus
• Electron Cloud
Different types of orbitals have different
shapes and different levels of energy.