Embed Size (px)
Citation preview
Origins of the Atomic Theory
Physical Science
2009-2010
psquires
The Development of the Atomic Theory
Democritus and Dalton:
atomic theory
DemocritusGreek philosopher ~ 300 BCLimit to “smallness”All matter consists of tiny,
indestructible particles called atoms
Atomos – indestructible
John DaltonFirst serious atomic theory
English scientist Studied the properties of gases“Reinvented” the idea of atomsPublished in 1803
1. Elements are composed of tiny, discrete, particles called atoms.
Dalton’s atomic theory - 1803
2. Atoms are indivisible and indestructible and do not change their identity during reactions.
Dalton’s atomic theory - 1803
3. Atoms of the same element are identical in mass and chemical and physical properties. Atoms of different elements are different.
Dalton’s atomic theory - 1803
4. Atoms combine to form compounds in simple, whole-number ratios.
Dalton’s atomic theory - 1803
Law of Definite Proportions
5. Atoms combine in different ratios to make two or more compounds.
Dalton’s atomic theory - 1803
Law of Multiple Proportions
The Development of the Atomic Theory
Thomson: CRT’s and the electron
J. J. ThomsonCathode rays - cathode ray tubeAttracted to positive electrodeThought they might be atomsHad same charge to mass ratio
regardless of metal in the cathodeParticle must be common to all
matter, a subatomic particle
Cathode Ray Tube
High voltage
Cathode Anode
+
It was also used by J. J. Thomson
That particle was called the …
The electron
The electronThe electronThe electron
The electron
The ElectronDiscovered in 1897By J. J. Thompson
J. J. ThomsonIf that were the case, then the electron would be much smaller than the smallest atom,
… showing for the first time that matter is made up of
particles smaller than atoms.
Thomson tried to measure the fundamental charge on the electron.
The Development of the Atomic Theory
Rutherford: “Gold Foil Experiment”
The Gold Foil Experiment
Top View
Side View
The Gold Foil Experiment
Alpha particle source
Gold foil Fluorescentdetector
ZnS
All of this was in a vacuum chamber.
The Gold Foil ExperimentMost of the
particles went…
…straight through the gold foil, undeflected.
The gold is mostly “empty space.”
Alpha Particles
Alpha particles are helium nuclei.
++
Two protons and
two neutrons.
The alpha particle is positively charged.
Gold Foil Experiment: Results
+
source
Small, dense, positively charged
nucleus of gold
Rutherford’s Nuclear Atom
Alpha particles were repelled by…
… a small, dense, positively charged nucleus.
Almost all the mass of an atom is in the nucleus.
Electrons are located outside the nucleus.
Niels Bohr
Electrons travel in fixed orbits around the atom’s nucleus.
Bohr also described the way atoms emit radiation by suggesting that when an electron jumps from an outer orbit to an inner one, that it emits light.
Later other physicists expanded his theory into quantum mechanics.
This theory explains the structure and actions of complex atoms.
Bohr diagram
