The Philosophers of Chemistry

1
The Philosophers of Chemistry
2
Democritus

• 460-370 B.C
• Ideas
• Matter is composed of empty space and atoms move
  through it
• Different properties of matter are due to the
  size, shape, and movement of atoms
• Changes in matter result from changes in
  arrangements of atoms (not the changes of atoms)
• Challenged by Aristotle because Aristotle
  didnt think that nothingness/empty space could
  exist.
• Couldnt prove that empty space did exist, so his
  theory failed

3
John Dalton

• 1766-1844
• School teacher in England
• Began studying atoms in 1800
• 1808 published Daltons atomic theory
• All matter is composed of atoms
• Atoms cannot be created, divided into smaller
  particles, or destroyed.
• All atoms of a given element are identical in
  size, mass, and chemical properties.
• Different atoms combine to form compounds.
• In a chemical reaction, atoms are
  separated/combined/rearranged.
• Work was inaccurate atoms CAN be divided!

4
Sir William Crookes

• English physicist
• Working on a vacuum pump that passed electricity
  through glass tubes
• Noticed that the electricity was being passed in
  rays from the cathode (negative) end. Called it
  the CATHODE RAY TUBE (CRT).
• CRTs are used in computers and televisions today
  (images formed as radiation from the CRT hits
  light producing chemicals on the coated back
  sides of the screen.
• These particles coming from the negative end are
  called ELECTRONS

5
J.J. Thomson

• Experimented with CRT to determine the mass of
  the negative particle that Crookes discovered
• Was able to determine the charge-to-mass ratio
  of the charged particle
• much less than the mass of a Hydrogren atom (the
  lightest known atom)
• This proved part of Daltons atomic theory WRONG
  there are particles smaller than an atom
• Wasnt widely accepted because Daltons theory hd
  become very popular by now
• Credited with discovering the first subatomic
  particle the electron

6
J.J. Thomson

• Plum Pudding Model
• Spherically shaped atom with positive charges
  distributed evenly throughout
• Similar to chocolate chip cookie dough

7
Robert Millikan

• 1868-1953
• Discovered the charge of an electron in 1909
• Negative
• His estimates of the charge (-1) is within 1
  accurate of todays accepted charge of an
  electron

8
Ernest Rutherford

• 1871-1937
• Won the Nobel Prize 1908 (in Chemistry)
• Interested in studying how positively charged
  particles interact with solid matter

• Gold Foil Experiment
• Shot a beam of alpha particles (positive charge)
  at a piece of gold foil
• According to Plum Pudding Model, these beams
  should pass straight through because the positive
  charged particles in the gold foil were evenly
  spread out in the foil
• Some particles bounced back (oops!)
• This is like shooting an artillery shell at a
  piece of notebook paper and having it bounce back
  at you!
• This disproves Thomsons plum pudding model
  because the particles were bounced back and not
  passed through

9
Rutherfords Gold Foil Experiment
10
Ernest Rutherford

• Developed his own atomic model
• Had a dense NUCLEUS in the center
• Electrons moved through empty space around
  nucleus
• This model still couldnt account for atomic mass
• 1919 Discovered the proton
• Has a 1 charge

11
James Chadwick

• 1932 - Showed that the nucleus contains another
  particle the NEUTRON
• Mass equal to proton
• Has NO charge (is neutral)

12
Bohrs Model of the Atom

• Neils Bohr Model
• Electrons rotate around center (nucleus) in
  orbitals

Protons
Electrons
Neutrons
13
Electron found not to exist??

• DeBroglie found that electrons act as waves
• (page 129 Text)
• If they are a wave
• how can they
• be a particle??

14

• As if that is not enough.

15
Heisenberg Uncertainty Principle

• States that it is
• impossible to
• know precisely
• both the velocity
• and position of a particle at the same time.

16
(No Transcript)
17
Schrodinger Wave Equation

• His equation treated an electron as a wave.
• And it did just as good a job as the old models
  in describing Hydrogen.
• Now called the Quantum Mechanical Model of the
  Atom.

18
Schroedingers ideas

• Predicts a 3-D area
• surrounding the nucleus
• where the electron may
• be found.
• Called an orbital
• See Ch. 5 notes for more.

19
Through the looking glass..

• As time goes on the atom gets curiouser and
  curiouser.

20

• Revised 9/2005