History Of The Atom Flashcards
Contribution of The Greeks
Proposed matter was made of small indivisible particles.
Contribution of John Dalton.
Atomic Theory
- Matter is made up of small particles called atoms.
- Atoms are indivisible
- Atoms of the same element have identical atomic mass.
Contribution of William Crookes.
Discovered Cathode Rays.
Consist of very small particles.
Describe how cathode rays were discovered.
-Passed electric current through a vacuum tube.
Observed fluorescence on the glass at the far end of the tube and a shadow of an anode metal cross in the tube was visible.
Concluded that radiation/rays were coming from the cathode that was being blocked by the anode metal cross. Called these rays cathode rays.
Describe how Crookes discovered Cathode rays consist of very small particles.
Passed electric current through a vacuum tube containing a light paddle wheel. The paddle wheel moved and always away from the cathode.
Concluded: Cathode rays must consist of very small particles that were able to move a paddle wheel.
Who coined the term electron?
George Stoney.
Contribution of JJ Thompson.
- Discovered the electron.
- Discovered that electrons are negatively charged.
- Calculated the (energy/mass) ratio of the electron.
Describe how it was discovered that the electron was negatively charged.
JJ Thompson passed cathode rays through a small hole in the anode to obtain a narrow beam which he then passed between two parallel plates. The beam could be seen from fluorescence at the end of the tube. When the plates were not charged the beam passed straight through but if the plates were charged the cathode rays were deflected towards the positively charged plates.
Concluded: Cathode rays consist of negatively charged subatomic particles called electrons.
Describe the plum pudding model.
A sphere of positive charge with negative charges called electrons imbedded at random.
Give five properties of cathode rays.
Consist of negatively charged electrons.
Can be deflected by electric and magnetic fields.
Can be detected by fluorescence on a glass tube.
Can cause a small paddle wheel to move.
Travel in straight lines because of the shadow from the cross.
Contribution of Robert Millikan.
Determined the size of the charge of the electron. Known as Millikan’s oil drop experiment.
Contribution of Rutherford.
Discovered the nucleus.
Discovered protons.
How did Rutherford discover protons? Don’t include conclusion.
Bombarded a thin gold foil with alpha particles.
Observed: Expected: Most of the alpha particles went straight through the gold foil undeflected. Unexpected: Some alpha particles were deflected slightly and some at larger angles, others repelled completely back along their own path. This disproved the plum pudding model of an atom.
What did Rutherford conclude after his experiment?
Atoms are mostly empty space.
That there is an area in the centre of an atom which is very small, dense and positively charged. (The Nucleus) Electrons must reside in an electron cloud surrounding the nucleus.
Contribution of James Chadwick. Why did he think there was something else in the nucleus?
Discovered the Neutron. Because the protons in the nucleus would repel each other.
