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History of the Atom
Transcript of History of the Atom
Democritus' Theory (460 BCE)
Atoms are indestructible, invisible, and homogeneous.
Atoms of different substances were of different size, shape, mass, or arrangement.
Atoms of solids were small and pointy
Atoms of liquids were large and round
Atoms of oils were small and could slip through each other.
Aristotle did not believe in the atom, but four main elements: Earth, Water, Air, and Fire.
He believed that all substances contained these elements in different proportions.
Because his theory was more applicable, his was accepted over Democritus' for 2,000 more years.
1. All matter consists of particles.
2. Atoms are indestructible and unchangable.
3. Elements are defined by their atomic mass.
4. When element react, their atoms combine in whole-number ratios.
5. Elements are pure because all atoms of an element are identical.
6. Compounds consist of different elements combined together.
Millikan's Oil Drop Experiment
Atomizer that sprays tiny droplets of oil
short focal distance telescope for viewing droplets
two plates, one negative, one positive above and bellow the chamber
dc supply is attached to plates
Date:1910 & again on 1913
oil drops fall through the hole in the upper plate
x-rays illuminate the bottom chamber, ionizing the air
the droplets go through the air; electrons accumulate over the droplets and a negative charge is acquired
voltage from the dc supply is applied
by adjusting the voltage, drops can be suspended in mid-air
RESULTS: Millikan confirmed that the charge of a single electron is 1.592x10^-19 Coulombs
Gold Foil Experiment
Before Rutherford, the common model for an atom consisted of many, free flowing electrons called the Plum Pudding Model.
Rutherford proved that there is a nucleus to an atom and theorized what it's composition would be.
If the Plum Pudding Model was correct, than atoms were mostly empty space with a few dangling electrons here and there.
So Rutherford devised and experiment to confirm this model.
The Gold Foil Experiment
The experiment consisted of shooting alpha particles at a piece of gold foil and seeing if there is deflection.
He was expecting something like this.
Before Bohr, atoms were thought to orbit around the atomic nucleus in a curved path and eventually spiral into the nucleus.
Why was Aristotle's theory accepted?
The equipment necessary to prove Democritus' theory was not available at the time. Also, Aristotle's theory of the elements could be more easily "observed", as his concept conveyed matter being made up of tangible objects; whereas the atom was impossible to observe at the time.
electrons are restricted to quantized (fixed) orbits
electrons can jump between orbits by absorbing or emitting a photon with a precise wave length
Gold Foil Experiment
Significance of Dalton's Theory
First theory to view compounds as combination of elements.
First proposed what is now the Law of Multiple Proportions
Proposed that new substances were formed by chemical reaction.
The Actual results were surprising.
What Remains Valid?
Much of Dalton's theory remains valid.He published the Law of Multiple Proportions, which remains today and also was correct in stating that elements are defined by their atomic mass. Also, he was correct in stating that elements are pure because their atoms are identical.
Bohr's model of the atom is also known as the "planetary model" due to its similarity to the solar system. The nucleus of the atom in Bohr's model consists of protons and neutrons clustered together. The electrons in Bohr's model orbit around the atomic nucleus in elliptical patterns the same way planets orbit the sun.
Gold Foil Experiment
For most of the particles, they passed right through the gold with little to no deflection. But some of the particles were deflected a few degrees. Even more surprising were the handful of particles being deflected 90 degrees or more.This proved that atoms are more than just electrons.
The New Model
Rutherford theorized a new model for the atom, it had a nucleus with protons and Neutrons though neutrons were yet to be proven.
J. J. Thomson
James Chadwick proved the existence of Neutrons (neutral particles) in an atom's nucleus in 1932.
What Are Cathode Rays
Cathode rays are a stream of charged particles.
The particles carried a negative charge.
These negatively charged particles are part of all matter.
J.J. Thomson's Experiment
J.J. Thomson Experiment Continued
Board of Trustees at the University of Illinois :: Department of Physics :: College of Engineering :: University of Illinois at Urbana-Champaign. (n.d.). Q & A: Bohrs Planetary Model. Physics Van. Retrieved October 17, 2014, from https://van.physics.illinois.edu/qa/listing.php?id=1279
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Ernest Rutherford. (n.d.). Biography. Retrieved October 16, 2014, from http://www.thefamouspeople.com/profiles/ernest-rutherford-140.php
James Chadwick. (n.d.). James Chadwick. Retrieved October 16, 2014, from http://chemed.chem.purdue.edu/genchem/history/chadwick.html
Joseph John Thomson | Chemical Heritage Foundation. (n.d.). Joseph John Thomson | Chemical Heritage Foundation. Retrieved October 14, 2014, from http://www.chemheritage.org/discover/online-resources/chemistry-in-history/themes/atomic-and-nuclear-structure/thomson.aspx
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Physical Sciences : Amrita Vishwa Vidyapeetham Virtual Lab. (n.d.). Physical Sciences : Amrita Vishwa Vidyapeetham Virtual Lab. Retrieved October 16, 2014, from http://amrita.vlab.co.in/?sub=1&brch=195&sim=357&cnt=1
Robert A. Millikan. (n.d.). Robert A. Millikan. Retrieved October 14, 2014, from http://www.nndb.com/people/771/000091498/
The Oil Drop Experiment. (n.d.). The Oil Drop Experiment. Retrieved October 14, 2014, from http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Ryan_McAllister/Slide3.htm
This Month in Physics History. (n.d.). This Month in Physics History. Retrieved October 16, 2014, from http://www.aps.org/publications/apsnews/200608/history.cfm
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The hole in the anode produces a thin cathod ray. A phosphor coating allows the ray to be determined where it strikes the tube. When a magnetic field is added, the beam is deflected proving these are charged particles. Then when a positively charged plate by an electric field is placed, the particles deflect again validating that they are negatively charged particles.
J.J. Thomson's Discovery
His observations led him to the discovery of the electron. This disproved Dalton's theory that atoms were not divisible into smaller subatomic particles.
"Plum Pudding" Model
This model of the atom is spherical shaped with a uniform positive charge where the separate electrons occupy.