HISTORY OF THE ATOM

1
HISTORY OF THE ATOM
2
Aristotle

• 400 BC
• - Claimed that there was no smallest part of
  matter
• - Thought different substances were made
  up of proportions of fire, air, earth, and
  water.

3
Democritus
400 BC - Matter could be subdivided into
smaller and smaller particles until a basic
part of matter could not be divided further
- He called these particles atoms, which
means indivisible in Greek.
4
John Dalton1766-1844

• Daltons Atomic Theory
• 1. All matter is made of atoms. Atoms are
  indivisible and indestructible.
• 2. All atoms of a given element are identical in
  mass and properties
• 3. Compounds are formed by a combination of two
  or more different kinds of atoms.
• 4. A chemical reaction is a rearrangement of atoms

5
Sir William Crookes1832-1919

• 1870s
• - Studied the effects of magnetic and electric
  
• fields on the cathode ray using a
  cathode ray tube (CRT)

6
He discovered cathode rays had the following
properties 1. Travel in straight lines from the
cathode 2. Cause glass to glow 3. Transmit a
negative charge to objects they
strike 4. Are deflected by electric fields and
magnets to suggest a negative charge 5.
Cause pinwheels in their path to spin
indicating they have mass
7
What is a CRT?

• 1. Consists of a gas-filled glass tube with 2
  metal plates
• a. 1 negatively charged (the cathode) and the
  other positively charged (the anode)
• When a very large voltage is placed across the
  electrodes
• a. the neutral gas inside the tube will ionize
  into a plasma
• b. The current will flow as electrons travel
  from the cathode (-) to the anode ()
  called a cathode ray.

8
J.J. Thomson1856-1940

• 1897
• - Discovered the electron using the CRT
• Found that the cathode ray was attracted to
  a positively charged metal plate
• Cathode ray must be negative because
  opposites attract.

9
Thomsons Plum Pudding Model

• Atoms are neutrally charged (0)
• - there must be a positive charge in the atom,
  too if there are electrons (-)
• Made a speculation at what an atom might look
  like
• - a cloud of positive electricity with
  electrons stuck in the cloud (chocolate
  chip cookie!)

10
R.A. Millikan(1868-1953)
Oil drop experiment measured the charge of an
electron - Electrons charge 1.602 x 10-19
coulombs - Electrons mass 9.11 x 10-28 grams
11
Ernest Rutherford 1871-1937
Used a gold foil experiment to determine -
the tiny center of the atom is dense and
positively charged - Discovered
the nucleus The proton () the
nucleus has almost all of the mass of an
atom most of the atom is empty space
assumed electrons were outside the nucleus
and create most of the volume of the atom
12
The Gold Foil Experiment
"It was as incredible as if you fired a 15-inch
shell at a piece of tissue paper and it came back
and hit you."
13
Niels Bohr1885-1962
Bohr expanded upon Rutherfords theory by
proposing that 1. electrons travel only in
certain successively larger orbits. 2.
the outer orbits could hold more electrons
than the inner ones, and that these outer 3.
the orbits determine the atom's chemical
properties
14
The Bohr Model
15
James Chadwick1891-1974
1932 Discovered - the neutron in the nucleus
(with the proton) - the neutron has mass, but
no (0) charge
16
Louis de Broglie1892-1987
Discovered 1. All particles, including light
electrons show both wave behavior
particle behavior 2. Electrons do
not travel in any specific pattern - The exact
location cannot be determined - There is a
high probability of finding electrons in
regions (orbitals)
17
Werner Heisenberg1901-1976
Determined the Uncertainty Principle - Any
effort to locate an electron will cause the
electron to move, therefore, it is
impossible to locate electrons