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Unit 2
2.1 – History of the
Atomic Model
Atomic Structure
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Democritus
400 B.C.
A. He was a Greek philosopher of
science.
B. First to use the term “atom” to
describe the basic particle of
nature.
1. “atom” means “indivisible”
2. Atom – the smallest particle of an
element that still retains the
chemical properties of that
element.
John Dalton
1808
• He was an English
schoolteacher.
• He was the first to propose an
“Atomic theory” that states
the 5 following statements:
John Dalton
1808
1. All matter is composed of extremely small particles
called “atoms”.
2. Atoms of a given element are identical in size, mass,
and other properties; atoms of different elements
differ in size, mass, and other properties.
a. This has since been modified based on Isotopes and ions.
3. Atoms cannot be subdivided, created, or destroyed.
a. This has since been modified based upon current studies
in quantum physics.
b. Such examples include muons and quarks.
John Dalton
1808
4. Atoms of different elements combine in simple
whole-number ratios to form chemical compounds.
5. In chemical reactions, atoms are combined,
separated, or rearranged.
J.J. Thomson (1897)
with the Cathode Ray Tube
J.J. Thomson (1897)
with the Cathode Ray Tube
A. He was an English Physicist.
B. He worked with glass gas-filled tubes referred
to as Cathode-Ray tubes.
1. The glass tubes were filled with a gaseous
element under low pressure.
2. He then passed an electrical current using a
battery and wires.
J.J. Thomson (1897)
with the Cathode Ray Tube
a. The electrical current caused the gas within the tube to glow
with a beam (“ray”) within the tube to intensely glow within
the tube.
i.
Magnets could make the “ray” move/deflect in various
directions.
The ray is being deflected by the negative charge of the
magnet.
Negative charge repels/deflects like negative charges.
The ray is made of negatively charged particles, Thompson
called electrons (since they were associated with the electrical
current.)
ii.
iii.
iv.
•
The symbol for an electron is: e-
J.J. Thomson (1897)
with the Cathode Ray Tube
b. The electrical current came into the chamber (by
a wire) at the cathode end. (The end where
electricity enters the tube.)
c. The electrical current left the tube on the anode
end. (The end where the electricity goes back
into the wire.)
d. Hence the term Cathode Ray tubes.
Discovery of the Electron
C. Further investigations using different elements
in Cathode-Ray tubes confirmed that every
elements atoms possess electrons.
Actual Cathode Ray in action
Thomson’s Atomic Model
D. He proposes the “Plum Pudding” model of atoms.
1. It states that negatively charged electrons are
evenly placed inside a positively charged mass.
2. Thomson believed that the electrons were like
plums embedded in a positively charged “pudding,”
thus it was called the “plum pudding” model.
Robert A. Millken
1909
A. He was an American
Physicist.
B. He was the first to
measure the charge
and mass of an
electron.
Millikan’s Oil Drop Experiment
Helps to calculate the mass & charge of an
electron
How it worked inside
Robert A. Millken
1909
1. Electron charge = 1.602 x 10-19 Coulombs.
a. This is an extremely small quantity of energy.
2. Electron mass = 9.11 x 10-31 kg
a. Electrons are 1/1837th the mass of a single
proton or neutron.
b. This is a very, very, very small amount and size.
Robert A. Millken
1909
C. Milliken’s experiments allow for 2 inferences
(conclusions based upon evidence and
reasoning) to be made:
1. Because atoms, in the natural state, are
electrically neutral, they must also contain an
equal amount of positively charged particles.
2. Because electrons have so little mass, atoms
must contain other particles with much greater
mass (protons & neutrons).
Ernest Rutherford (1911)
on New Zealand Money
Hans Geiger (1911)
Ernest Marsden (1911)
Rutherford’s Gold Foil Experiment
A. Geiger and Marsden were students of the
New Zealand Physicist.
B. They performed the Gold Foil Experiment.
C. They used high-energy alpha particle
radiation( 2 protons & 2 neutrons ejected
from a decomposing, radioactive element) to
bombard a piece of gold foil that is
surrounded by a fluorescent screen.
Rutherford’s Gold Foil Experiment
1. As alpha particles struck the fluorescent
screen, they would produce a small
detectable burst of light.
2. As the experiment was running, they
detected light burst mainly behind the gold
foil, but also occasionally all around the ring.
Rutherford’s Gold Foil Experiment
a. These bursts of light around the ring were
because of the positively charged alpha particles
been deflected by positively charged particles in
the atoms of the foil.
b. The particles became known as protons.
i.
Just as with the electrons, positive charges
repel/deflect like positive charges.
Rutherford’s Gold Foil Experiment
a. As most of the bursts of light occurred behind
the gold foil, they concluded that the majority of
space in an atom is “empty space”. The alpha
particles travelled through and never hit
anything.
b. Rutherford proposes the idea of the neutrally
charged neutron particle in 1920.
Rutherford’s Gold Foil Experiment
 Alpha
() particles are helium nuclei
 Particles were fired at a thin sheet of gold foil
 Particle hits on the detecting screen (film) are recorded
Rutherford’s Findings
 Most of the particles passed right through
 A few particles were deflected
 VERY FEW were greatly deflected
“Like howitzer shells bouncing off of tissue paper!”
Conclusions:
 The nucleus is small
 The nucleus is dense
 The nucleus is positively charged
Atomic Particles
Niels Bohr
1913
A. He was also a student
of Rutherford’s.
B. He proposed the Bohr
model of an atom.
1. The electrons move in
a circular pattern
around the positively
charged center.
a.
Much like the planets
revolve around the sun.
Bohr Model of an Atom
Erwin Schrödinger
1926
• He was an Austrian Physicist.
• He helped develop the
Quantum Theory of Atoms
– This Theory tries to describe,
by mathematics, the wave-like
properties of electrons & other
very small particles.
Schrödinger
• This re-enforces that electrons travel in orbitals.
• Orbital is defined as a 3- Dimensional region
around a nucleus that indicates the probable
location of a single electron within an orbital
Werner Heisenberg
1927
• He was a German Physicist.
• He calculated that electrons
and photons have about the
same amounts of energy.
– Photons are used to help detect
the presence of electrons.
– When they collide, the electron is
deflected in a random direction.
Heisenberg’s Uncertainty Principle
• This states that it is impossible to determine
exactly both position and velocity simultaneously
of an electron in an orbit.
Dmitri Ivanenko (1930)
Victor Ambartsumian (1930)
Ivanenko & Ambartsumian
1930
A. These gentlemen were Russian Physicists.
B. They proposed a model of the nucleus of an
atom being composed of positively charged
protons and neutral charged particles
(neutrons).
Ivanenko & Ambartssumian Model
of a Nucleus
James Chadwick
1932
A. He was an English
Physicist.
B. He also studied under
Rutherford.
C. He proved that the
nucleus is definitely
composed of protons
and neutrons through
his experiments with
alpha particle radiation.
His Experiment using alpha particle
radiation