the atom unit 3, presentation 1. history of the atom not the history of atom, but the idea of the...
TRANSCRIPT
The Atom
Unit 3, Presentation 1
History of the atom
Not the history of atom, but the idea of the atom– The atom was not “discovered” until recently
Original idea Ancient Greece (400 B.C.)– Proposed by lesser known scientists
History of Atom
The looked at a beach– Made of sand
If you cut sand – smaller sand
What is the smallest possible piece?What is the smallest possible piece? Atomos - not to be cutAtomos - not to be cut
Another Greek
Aristotle - Famous philosopher All substances are made of 4 elements
– Fire – Hot, dry– Air - light– Earth - cool, heavy– Water - wet
These are blended in different proportions to get all substances
Who Was Right?
Greek society class based Thought to be beneath famous scientists to work
with hands– Aristotle did not experiment
Greeks settled disagreements by argument– Aristotle was more famous– He won
His ideas carried through middle ages.– Alchemists change lead to gold
Who’s Next?
Late 1700’s - John Dalton- England He was a teacher
– summarized results of his experiments and those of other’s
In Dalton’s Atomic Theory– Combined ideas of elements with that of atoms
Dalton’s Atomic Theory
All matter is made of tiny indivisible particles called atoms.
Atoms of the same element are identical, those of different atoms are different.
Atoms of different elements combine in whole number ratios to form compounds
Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.
Law of Definite Proportions
Each compound has a specific ratio of elements
It is a ratio by mass Water is always 8 grams of oxygen for each
gram of hydrogen
Law of Multiple Proportions
If two elements form more that one compound, the ratio of the second element that combines with 1 gram of the first element in each is a simple whole number.
What?
Water is 8 grams of oxygen per gram of hydrogen.
Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen.
2
8 X2Y16 X 8 Y+
Parts of Atoms
J. J. Thomson - English physicist. 1897 Made a piece of equipment called a cathode
ray tube. It is a vacuum tube
Thomson’s Experiment
Voltage source
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Vacuum tube
Metal Disks
Thomson’s Experiment
Voltage source
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Thomson’s Experiment
Voltage source
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Thomson’s ExperimentThomson’s Experiment
Voltage source
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Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
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Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
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Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
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Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end
Thomson’s ExperimentThomson’s Experiment
Voltage source
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Voltage source
Thomson’s Experiment
By adding an electric field
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field By adding an electric field
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field By adding an electric field
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field By adding an electric field
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field By adding an electric field
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field By adding an electric field
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Voltage source
Thomson’s ExperimentThomson’s Experiment
By adding an electric field he found that the By adding an electric field he found that the moving pieces were negative moving pieces were negative
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Cathode Ray Tube
Thomson’s Model
Found the electron But he couldn’t find
positive (for a while) Said the atom was like
plum pudding A bunch of positive
stuff, with the electrons able to be removed
Other pieces
Proton - positively charged pieces 1840 times heavier than the electron
Neutron - no charge but the same mass as a proton.
Where are the pieces?
Rutherford’s experiment
Ernest Rutherford English physicist. (1910) Believed in the plum pudding model of the atom. Wanted to see how big they are Used radioactivity Alpha particles - positively charged pieces given off
by uranium Shot them at gold foil which can be made a few
atoms thick
Lead block
Uranium
Gold Foil
Fluorescent Screen
When alpha particles strike a fluorescent screen, it glows.
1. atoms positive charge is concentrated in the nucleus2. proton (p) has opposite (+) charge of electron (-)3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)
particle velocity ~ 1.4 x 107 m/s(~5% speed of light)
(1908 Nobel Prize in Chemistry)
He Expected
The alpha particles to pass through without changing direction very much
Why?– The positive charges were spread out evenly.
Alone they were not enough to stop the alpha particles
What he expected
Because
Because, he thought the mass was evenly distributed in the atom
Because, he thought the mass was evenly distributed in the atom
What he got
How he explained it
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Atom is mostly empty Small dense,
positive piece at center
Alpha particles are Alpha particles are deflected by nucleus deflected by nucleus if they get close if they get close enoughenough
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Modern View
The atom is mostly empty space
Two regions Nucleus- protons and
neutrons Electron cloud-
region where you might find an electron
Density and the Atom
Since most of the particles went through, it was mostly empty.
Because the pieces turned so much, the positive pieces were heavy.
Small volume, big mass, big density This small dense positive area is the nucleus
atomic radius ~ 100 pm = 1 x 10-10 m
nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m
Rutherford’s Model of the Atom
“If the atom is the Houston Astrodome, then the nucleus is a marble on the 50-yard line.”