Transcript of Atomic Theory “History of... ”. The Ancient Greeks Democritus and other Ancient Greeks were the...
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- Atomic Theory History of...
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- The Ancient Greeks Democritus and other Ancient Greeks were the
first to describe the atom around 400 B.C. The atom was natures
basic particle that makes up all matter. Since there was no
scientific evidence, many others believed Aristotles ideas instead,
since he was more popular.
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- The Ancient Greeks The Greek philosopher Aristotle did not
believe in atoms, and instead believed all matter was continuous.
He thought the elements were Earth, Fire, Wind and Water this is
not correct. Aristotles belief was accepted for ~2000 years, as
there was no evidence to support either theory.
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- What changed? In the 1700s scientists' began to seriously
question whether Aristotle was correct. Bernoulli (1738), Dalton
(1808) and Faraday (1834) began studies which proved him wrong
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- John Daltons Atomic Theory (1808) He proposed a simple model of
an atom that was small but indivisible and uniform throughout. His
atomic theory was... 1)All matter is composed of extremely small
particles called atoms. 2)Atoms of an element are identical to each
other and different from atoms of other elements.
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- Daltons Theory continued 3) Atoms cannot be created or
destroyed. Atoms are combined, separated or rearranged when
chemical reactions occur. 4) Atoms of different elements combine in
simple whole-number ratios to form chemical compounds.
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- Daltons Model Daltons model of the atom is a small, solid, and
indivisible particle. (Ball- Bearing Model similar to a
marble)
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- But Dalton wasnt entirely correct Today we know that. 1.Atoms
are divisible into even smaller particles 2.Atoms of the same
element can have different masses. (called isotopes)
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- JJ Thompson (1897) Conducted the Cathode-Ray Tube Experiment
Thomson concludes that all cathode rays are composed of identical
negatively charged sub-atomic particles, which he calls electrons.
Proposed the Plum-Pudding Model of an atom.
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- Plum-Pudding Model Thomsons experiments show that (1) atoms are
divisible and (2) they include negatively charged electrons.
Thomsons Plum-Pudding Model of the Atom has negative particles
spread evenly throughout a solid, positively charged sphere.
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- Thompsons Experiment Passed electric current through a tube.
The rays were deflected away from magnetic fields and negatively
charged objects. Demo
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- Ernest Rutherford (1911) Conducted the Gold-Foil Experiment
Positively charged alpha particles were fired at a thin piece of
gold foil. About 1 out of every 20,000 particles were actually
deflected back toward the source. Video ClipVideo Clip
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- Ernest Rutherford Concludes that there must be a very small and
densely packed region of positive charge in the atom that would
repel these alpha particles. Discovered nucleus and credited with
discovery of proton
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- Rutherfords model: He proposed a model of the atom where the
negatively charged electrons orbit a small, dense, positively
charged nucleus.
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- Niels Bohr (1913) Bohr added the idea of quantized energy
levels to his Rutherfords model. This explained why different
elements give off different colors of light when heated in a
flame.
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- Weakness of Bohrs model His model was only accurate for
predicting the behavior of Hydrogen atoms
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- The Nuclear Model of the Atom Nucleus = very small and dense
center of the atom, where protons and neutrons are found. Protons
have a positive charge. Neutrons are electrically neutral.
Negatively charged electrons orbit the nucleus like the planets
orbit the sun. Most of the atom is empty space.
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- Subatomic Particles ProtonNeutronElectron Charge+0 Mass*1 amu ~
0 LocationNucleus Electron Cloud *Mass of protons and neutrons =
1.67 x10 -27 kg compared to the electrons mass of 9.109 x10 -31
kg