Atomic Structure

1
Atomic Structure

• The Work of Crookes, Thomson and the discovery of
  the Electron

2
Sub-atomic Particles

• The definition of Element
• pure substance that cannot be broken down to
  simpler substance by ordinary chemical means
• the definition of Atom
• the smallest unit of matter that can take place
  in a chemical change (a particle of an element)

3
Smaller than atoms?

• These two definitions would suggest that there is
  nothing smaller than an atom and that atoms
  cannot be broken into simpler things.
• This isnt truescientists have discovered
  particles within atoms, the so-called
  sub-atomic particles
• Electrons, Protons Neutrons

4
The sub-atomic particles

• Protons
• Electrons
• Neutrons

• Charge of 1
• mass of 1 amu
• charge of -1
• virtually no mass (1/1863 amu)
• no charge
• mass of 1 amu

5

• The next few classes will go over, in detail, the
  advancements of science that have led to the
  discovery of these sub-atomic particles.

6
The discovery of the Electron

• In 1855 Heinrich Geissler developed an efficient
  gas discharge tube.
• In such tubes, scientists performed experiments
  to see what colour a gas glows when an electric
  current is passed through it

7
Discovery of Electrons

• Geisslers tube was used in experiments in which
  gas, at very low pressure had electric current
  passed through it.

• The electric current passed from the negative
  terminal (cathode) to the positive terminal
  (anode)
• Negative charges were released from the cathode
  and flowed to the anode

8

• As scientists continued to experiment with better
  and better tubes they learned that if you remove
  all of the gas from the tube, it glows green on
  one end (the anode)
• the theory was that this was as a result of the
  metal that the electrodes were made of
• this proved to not be the case, regardless of
  which metal usedthe tube still glowed green

9

• This fact led to the conclusion that something
  was being released from the negative electrode.
• These were called Cathode rayshence the tubes
  themselves came to be known as Cathode ray tubes
  or CRTs

10

• The best example of a CRT is the television
  screen or computer monitor you use every day.

11

• In the 1870s William Crookes did two experiments
  to prove that cathode rays
• 1. Came from the negative terminal and had a
  negative charge
• 2. Had mass
• he called cathode rays corpuscles or tiny bits
  of matter

12

• In 1894, Joseph J Thomson did an experiment that
  discovered
• 1. These corpuscles could have their path
  deflected by an electric field.
• 2.Their mass was very tiny compared to the mass
  of known atoms

13
J.J. Thomson the electron

• Thomson is generally credited with the discovery
  of the electron, although he never used the term
  himself.
• He also created a model of the atom that came to
  be known as the plum pudding model
• He suggested that negative particles floated
  randomly in a positive atom much like the raisins
  in a fruit cake.