1. The History of the Structure of the Atom

2. 440 B.C. Democritus
Atomos = uncuttable
Model = pebble (or marble)

3. 1802 John Dalton Atomic Theory: Atoms can’t be broken
Atoms of an element are exactly alike
Atoms of different elements are different
Atoms can combine to form compounds
Atoms of each element have a unique mass
Masses of elements in a compound are always in constant ratio.

4. 1879 Sir William Crookes Invents the Cathode Ray Tube Model:
The cathode ray tube (CRT) has a positive (+) side & a negative (-) side. Crookes put a cross- shaped object in the in the middle and two electrodes inside.
When he connected the electrodes to batteries the tube lit up with a greenish glow and the shadow of the cross shaped object appeared at the positive side. This showed that something was flowing from the negative side to the positive side. He didn’t know if it was light or particles.
When Crookes attached the CRT to batteries a greenish glow flowed from the (-) side to the (+) side
He wasn’t sure if it was particles or light flowing from the (-) to the (+) side

5. 1897 J.J. Thomson
Used the C.R.T. that Crookes used, but he added magnets.
Plum Pudding Model
J.J. Thomson repeated Crookes CRT experiment and conducted several variations of it.
When he placed magnets next to the CRT tube the beam of light bent in the direction of the magnet. Since light is not bent by magnets, he concluded that the beam was from negatively charged particles, electrons. He thought that most of the atom was (+) with (-) particles spread throughout. He came up with the “Chocolate Chip Cookie” or “Plum Pudding” model.
When Thomson placed magnets next to the CRT the green glow bent towards the magnet
Since light is not bent by magnets, he concluded that it was negatively charged particles.

6. 1911 Ernest Rutherford Gold Foil Experiment
Shot positively charged particles at a piece of gold foil. Most went straight through as expected. However some came straight back or were deflected at angles. He concluded that they had hit something w/a + charge. He thought that the atom had a positive core/nucleus that negative electrons orbit.
Shot positively charged particles at gold foil, most went straight through as expected but some came straight back or were deflected at angles.
Concluded that deflected/reflected particles had hit something with a positive charge.
Thought atom had a (+) core/nucleus that (-) electrons orbit.
* Draw and label picture of experiment showing paths of positively charged particles shot at gold foil.

7. 1922 Niels Bohr Atom has orbitals around a positive core.
Electron

8. 1930 Schrodinger
Devised an atom similar to Bohr, but included a cloud of negative particles instead of rings.

9. The structure of the atom today
Electron (-)
Neutron (0)
Proton (+)
Protons and neutrons make up the nucleus. Electrons orbit the nucleus.
The current model of the atom was the work of many individual scientists who took previous improvements and added to them.