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Atomic Theory
Atoms
All matter is composed of tiny particles
called atoms.
An atom is the smallest particle of an
element that retains the characteristics of
the element.
Evolution of the Atomic Model
Atoms are far too small to be seen even
with the most powerful electron
microscopes, so models of them have
been developed.
– Model: a verbal, mathematical or visual
representation of a scientific structure or
process
Early Days
The Four Element Theory: (lasted for about 2000
years!)
According to a Greek philosopher called Aristotle, all
matter was comprised of four elements: earth, water,
air and fire.
Early days still…
First mention of
atoms!!!
In 300 BC,
Democritus, a Greek
philosopher said
atoms were indivisible
particles. Smart guy.
Dalton’s Model of the Atom
In the early 1800’s, John Dalton ran an
electric current through water. Hydrogen
(H2) and oxygen (O2) bubbles were
formed.
a) They have very different physical and
chemical properties so their particles must
be different.
b) The volume of hydrogen was twice that
of oxygen.
Dalton’s Model of the Atom
I have a
theory…
With this and other information,
he formed the famous
Dalton’s Atomic Theory…
Dalton’s Model of the Atom
1. All matter is made up of small particles called
atoms.
2. Atoms cannot be created or destroyed, or divided
into smaller particles.
3. All atoms of the same element are identical in
mass and size. The atoms of one element are
different in mass and size from the atoms of
other elements.
4. Compounds are created when atoms of different
elements link together in specific proportions.
1. H2O
2. CO2
Dalton’s Model of the Atom
This was a huge development in science.
This model was excellent, but more
research showed that it required
amendments.
The Next Big Discovery
Following Dalton’s work, scientists studied
gases
– They applied electric current to different gases in a
tube and observed how the gases glowed
A gas-discharge tube uses
electrodes inside a low-pressure
gas to create light. When a high
potential difference exists between
the electrodes, the gas forms a
plasma. Free electrons flow
between the electrodes.
What is a gas discharge tube?
http://www.youtube.com/watch?v=GzMh4q-2HjM
The fact that cathode rays were deflected toward
the positive pole of the electric field applied
externally to the tube, demonstrated that they
had to be composed of negative charges.
The Electron
J.J. Thompson used the gas discharge
tube to obtain evidence of the existence of
the electron (1987).
Electrons had a negative charge.
The Electron
After experimenting with all sorts of
materials, he was surprised to see the
same ray produced each time.
These electrons were found in ALL atoms.
Electrons were the first subatomic
particles to be found.
New Atomic Model
JJ Thomson revised Dalton’s atomic
model.
He wanted it to reflect the presence of
newly discovered electrons.
He called it the “raisin bun model” where
the atom had electrons embedded within
it, like raisins in a bun….
Raisin Bun Model
Raisin Bun Model
Thomson knew the atoms were electrically
neutral.
 Positive charge = negative charge
These discoveries earned him the Nobel
Prize in 1906
Soon enough, one of Thomson’s students
proved him wrong!
The Discovery of the Nucleus
Ernest Rutherford was a student under
Thomson
Moved to Montreal
to study at McGill!
Holla!
Canada!
Rutherford’s Work
Radioactive elements were just recently
understood
– They give off rays of energy as they break
down = “radioactive decay”
Rutherford’s Work
Rutherford studied alpha particles which
are emitted from decaying radioactive
elements (ex. Radium, Polonium)
– Alpha particles are large and have a positive
charge
Rutherford’s Work
To further study the atom, he decided to
fire alpha particles at a thin sheet of gold
He expected the particles being shot at
the gold would pass through in a straight
line
Rutherford’s Work
But he was surprised!
– Most of the particles went straight through
– Some were deflected off course (weird)
– Some bounced right back at him (weirder!)
How in the world would he explain this??
Gold foil atoms
Rutherford’s ground-breaking
discovery
Because alpha particles are positive, for
them to bounce right back at him,
Rutherford proposed that a small region of
positive charge existed in the atom
– The alpha particles were repelled by the likecharged centre of the gold atoms
Rutherford’s ground-breaking
discovery
(+)
(+)
(+)
(+)
(+)
(+)
Rutherford’s ground-breaking
discovery
Rutherford named the centre of the atom
“nucleus”
Proposed the nucleus had a positive
charge
Proposed electrons orbited the nucleus,
like planets around the Sun
In Summary
Rutherford’s observations can be summarized
as follows:
Observations:
1. Most alpha particles
went straight through
the foil.
Conclusions:
Atoms are mostly empty
space.
2. Some were deflected a
little.
The nucleus is positive and
repelled the alpha
particles.
The nucleus is very tiny
compared to the size of
the whole atom.
3. A very few alpha
particles were
deflected straight
back.
Thomson’s model vs Rutherford’s model
Major advancement in the understanding
of the atom
The next find…
A Danish scientist (Neils Bohr) traveled to
England to work with now-famous
Rutherford
Famous
Curious
Bohr’s Contribution
Bohr figured Rutherford was mostly
correct
He experimented with hydrogen gas: when
he passed electricity through it, bands of
light appeared
– Light bands corresponded with quantity of
energy applied to the gas
– Unique bands of light were given off by ALL
elements
Bohr’s Contribution
Bohr reasoned that electrons moved in
fixed regions or energy levels
For an electron to move up a level, it
needed outside energy to “bump it up”
The amount of energy required to excite
an electron was called a “quantum”
Where can electrons be found?
In concentric “shells” around the nucleus…
… but in 3 dimensions.
Bohr-Rutherford Model of the Atom
Electrons cannot be ‘between’ shells
– Think of a person on a ladder. Can they
stand in between ladder rungs? No.
Bohr-Rutherford Model of the Atom
Electrons can be
bumped up after
absorbing energy
or they can “fall
down” if energy is
not sustained
But wait….
Can you guess what fundamental
problem there is with this idea??
– Hint: positive nucleus, negative charge orbiting
– How come the electrons don’t spiral right
into the nucleus since we know opposite
charges attract???
But what prevents electrons and
the nucleus from colliding??
After many years of research by many
scientists, the positive charge in atoms’
nuclei was identified as the “proton”
But, there had to be something else in the
nucleus…otherwise why don’t electrons
crash into the positive nucleus??
The neutral particle…
Scientists generally accepted that there
was a neutral particle in the nucleus too
But it wasn’t until 1932 (20 yrs after Bohr
discovered electron shells) that another
Brit, James Chadwick, proved that neutral
particles existed
– These were called “neutrons”
– He was awarded the Nobel Prize too
The Neutron
The neutron is neutral, has no charge
Neutrons in the nucleus prevent protons
from repelling each other
Neutrons in the nucleus exert the “strong
force” which overcomes the attraction
between protons and electrons (which
have opposite charges)
The 3 Subatomic Particles
http://www.mi.mun.ca/users/edurnfor/1100
/atomic%20structure/sld007.htm
– Good summary website
Mass Spectroscopy
“Average” Atomic Weight
Radioactive Decay