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Defining the Atom

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Katie Thomas

on 11 November 2016

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Transcript of Defining the Atom

Defining the Atom
Objectives:
Define the atom.
Distinguish between the subatomic particles in terms of relative charge and mass.
Describe the structure of the atom, including the locations of the subatomic particles

Main Idea:


An atom is made of a nucleus containing protons and neutrons; electrons move around the nucleus.
The atom is the smallest particle of an element that retains the properties of the element
What is an atom?
If they are so small, how do we know they exist?
Using a scanning tunneling microscope (STM) we are able to see individual atoms.
The Electron Tale
Late 1800's
Sir William Crookes discovers the cathode ray by accident while working in a darkened lab
What is a cathode ray?
A cathode ray is produced from a ray tube.
Cathode rays are a stream of charged particles
The particles carry a negative charge
The Atom
1897
J.J. Thomson used the cathode ray to identify the electron. This work won him the Nobel Prize 1906.
A series of cathode ray experiments studying the effects of both magnetic and electric fields on the ray.
These experiments determined the ratio of it's charge to it's mass.
He concluded that the mass of the charged particle was less than the mass of a Hydrogen atom
This proved that there was a particle smaller than an atom.
Late 1910's
Robert Millikan performs the oil drop experiment.
Be varying the intensity of the electric field Millikan could control the rate of the droplet's fall.
Found that the smallest common denominator was 1.602x 10^ -19 coulombs (the charge of one electron (now equated to a single unit, 1–)).
With the electron’s charge and charge-to-mass ratio known, Millikan calculated the mass of a single electron

1897
Oil Droplet Apparatus
What can we do with a cathode ray?
We know there is a subatomic particle, but what do we know about it?
The Nuclear
Majority
If matter is neutral, and the negative component is so small, what accounts for the rest of the mass of an atom?
J.J. Thomson tried to answer questions about the nature of atoms using a model.
His model, which would later be dubbed the plum pudding model, depicted the atom as a spherical shape composed of a uniformly distributed positive charge in which the negatively charged electrons resided.
Plum Pudding Model
Actual Bread Pudding
The Plum Pudding Model did not last long.
Ernest Rutherford studied how positively charged alpha particles interacted with solid matter, this is referred to as The Gold Foil Experiment.
Rutherford's Gold Foil Experiment
How the experiment was performed
By aiming the particles at a thin sheet of gold foil, Rutherford expected the paths of the alpha particles to be only slightly altered by a collision with an electron.
What did Rutherford learn from the experiment?
1911
Gold Foil Experiment Conclusions
Atoms are mostly empty space.
Almost all of the atom's positive charge and almost all of the atom's mass is contained in a dense region in the center of the atom called the nucleus.
Electrons are held within the atom by their attraction to the positively charged nucleus.
The repulsive force between the positively charged nucleus and the alpha particles caused the deflections
1920
Rutherford refined his concept of the nucleus and concluded that the nucleus contained positively charged particles called protons.
A proton is a subatomic particle that has a charge of 1+
The actual mass of a proton is 1.673 x 10 ^-24 grams
Rutherford's Nuclear Model
What caused this positive charge?
What about the remainder of the mass?
1932
The physicist James Chadwick, a co-worker of Rutherford, showed that there was another subatomic particle found in the nucleus.
A neutron is a subatomic particle that has a mass nearly equal to the proton, but carries no charge. ( 1.675x10^-24 grams)
Chadwick received the Nobel Prize in 1935 for proving the existence of neutrons.
The structure of the atom:

Atoms are spherically shaped.
Atoms are mostly empty space.
Atoms are made up of three fundamental subatomic particles: the electron, proton and the neutron.
Electrons travel around the positively charged nucleus.
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