atomic structure. there were many scientist that contribute to the development of the first models...
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Atomic Structure
There were many scientist that contribute to the development of the first models of matter
1. Aristotle
2. Democritus
3. John Dalton
Hypothesis – First Models Of Matter
4. J.J. Thomson
5. Rutherford
6. Bohr
Even today, basic principles of his theory are still true.
His model was modified to explain new observations of later chemists
Dalton’s Atomic Model
He is known as the father of the Atomic Theory.
He formulated four main points concerning atoms:
Developed by John Dalton
1) All matter is composed of tiny indivisible particle called atoms.
2) All atoms of the same element are identical (same).
3) Atoms of different elements are different.
4) During a chemical reaction atoms combine to form new products.
Memorize
J.J. Thomson’s Atomic Model
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Thomson discovered the electron while experimenting with cathode rays.
Electrons have a negative chargeThomson believed:
The atom was a positive sphere with electrons embedded in it like raisins in
a plum pudding.
It looks like a
chocolate chip
cookie!!!
Cathode Rays
Cathode rays are beams of tiny, negativelycharged particles called electrons that
flow through this tube.
A cathode ray tube is a sealed evacuated
glass tube.
J.J Thompson used these tubes to discoverthe various properties of cathode rays
(electrons)
Properties Of Cathode Rays
1. The cathode rays travel in a straight line.
2. The cathode rays drive a small propeller located in their path.
This tells us that cathode rays have mass.
Properties Of Cathode Rays
3. The cathode rays cause the shadow of the object in their
path to be projected on a fluorescent screen.
Properties Of Cathode Rays
4. The cathode rays are deflected towards the positively charged
plate.
This tells us that cathode rays are negative.
RUTHERFORD’S ATOMIC MODEL
HE BELIEVED:
the atom had a central positive nucleus surrounded by negative electrons floating around in a cloud like structure
Ernest Rutherford also developed his own atomic theory
Rutherford used the ideas of radioactivity to carry out experiments to help him better define the atomic model.
His experiment involved: Exploring matter by shooting alpha
particles against thin sheets of metal.
Rutherford’s Atomic Model
Radiumin Lead Block
AlphaParticle beam
Fluorescent Screen
Thin sheet ofGold foil
Fluorescent Screen
A FEW particles are sharply deflected
MOST particles passed through the gold foil & were not deflected
SEVERAL particles pass through the gold foil & were slightly deflected
Conclusions from Rutherford’s Experiment:
3 Main Observations
OBSERVATIONS CONCLUSIONS
1.MOST (almost all) went through the gold foil &
were not deflected
Atom contains a significant amount of empty space
2. FEW (not many) bounced directly back
The atom’s dense core (nucleus) must be very small
3. SEVERAL (some) passed through the gold
foil & were deflected
The nucleus must be positive (law of repulsion)
Alpha Particles
MOST went through empty space
FEW approach nucleus & bounced
back SEVERAL were deflected by the nucleus
Gold Atom
Rutherford Model
Rutherford proposed a new
model based on his findings
and Thomson's Model.
Rutherford’s Model placed electrons orbiting,
like a cloud, around a dense,
positively charged nucleus.
Electrons
Nucleus
++ +
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Which diagram is Thompson’s and which diagram is Rutherford’s?
Rutherford believed the nucleus was POSITIVE or NEGATIVE?
NEIL BOHR proposed modification to
Rutherford Model proposed that electrons can only
circle the nucleus at very specific distances (called orbits)
Bohr – Rutherford Atomic Model:
Bohr & Rutherford combined theirtheories to create the current
simplified atomic model
Thomson’s Model is known as the plum pudding model. He also discovered the electron.
Rutherford’s Model Positive nucleus surrounded by a fuzzy
cloud Electrons not organized
Bohr – Rutherford Modelstates that here is a nucleus that is surrounded by orbits (energy shells)