Transcript Atomic Structure

Unit 2
History of the Atomic Model
2.1
Atomic Structure
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Democritus
400 B.C.
Democritus
• 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
Dalton
• A. He was an English schoolteacher.
• B. He was the first to propse an “Atomic
theory” that contains the 5 following
statements:
– 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.
• This has since been modified based on isotopes and
ions
Dalton
• 3. Atoms cannot be subdivided, created, or
destroyed
– A. This has since been modified based upon
current studies in quantum physics. Such
examples include muons and quarks.
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.
Dalton’s view of atoms of elements
Can you see B. 1-4 in this picture?
J.J. Thomson (1897)
with the Cathode Ray Tube
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a
negatively charged particle.
Cathode ray tubes pass electricity through a gas that is contained at a very low
pressure.
Actual Cathode Ray in action
• 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 through
the gas using a battery and wires.
• a.The electrical current caused the gas within
the tube to intensely glow with a beam
• i. Magnets could make the “ray” move/deflect
in various directions.
• ii.The ray is being deflected by the negative
• charge of the magnet.
• iii.Negative charge repels/deflectslike negative
charges.
• iv.The ray is made of a negative charge that
Thompson called electrons (since they were
associated with the electrical current.)
• 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
Thomson’s Atomic Model
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
• He was an American Physicist.
• He was the first to measure the charge and
mass of an electron.
• The symbol for an electron is: e• Electron charge = 1.602 x 10-19 Coulombs.
• This is an extremely small quantity of energy.
• Electron mass = 9.11 x 10-31 kg
• This might be a good time to actually write the
number out to reinforce minutiae.
• Electrons are 1/1837th the mass of a single
proton or neutron.
• This is a very, very, very small amount and
size.
• Milliken’s experiments allow for 2 inferences
(conclusions based upon evidence and
reasoning) to be made:
• Because atoms, in the natural state, are
electrically neutral, they must also contain an
equal amount of positively charged particles.
• Because electrons have so little mass, atoms
must contain other particles with much
greater mass (protons & neutrons).
•
Millikan’s Oil Drop Experiment
Helps to calculate the mass & charge of an
electron
How it worked inside
https://www.youtube.com/watch?v=XMfYHag7Liw
Ernest Rutherford (1911)
on New Zealand Money
Hans Geiger (1911)
Ernest Marsden (1911)
• 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 was
surrounded by a fluorescent screen.
• As alpha particles struck the fluorescent
screen, they would produce a small detectable
burst of light.
• As the experiment was running, they detected
light burst mainly behind the gold foil, but also
occasionally all around the ring.
• 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.
• The particles became known as protons.
• Just as with the electrons, positive charges
repel/deflect like positive charges.
• 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” that the
alpha particles travelled through and never hit
anything.
• 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)
His comment on the structure of atoms. (Which
are mostly empty space.)
Bohr
• He was also a student of Rutherford’s.
• He proposed the Bohr model of an atom.
• The electrons move in a circular pattern
around the positively charged center. (Much
like the planets revolve around the sun.)
Bohr Model of an Atom
Dmitri Ivanenko (1930)
Victor Ambartsumian (1930)
Dmitri Ivanenko & Victor
Ambartsumian (1930)
• These gentlemen were Russian Physicists.
• They proposed a model of the nucleus of an
atom that is composed of positively charged
protons and neutral charged particles
(neutrons).
Ivanenko & Ambartssumian Model
of a Nucleus
James Chadwick
1932
• He was an English Physicist.
• 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