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100
492 B.C. - Democritus - Atomic Theory
- Theory: Matter cannot be divided into smaller and smaller pieces forever
- Atoms
- small, hard particles that were all made of teh same material but varied in shape and size
- infinite number and are always moving and capable of joining together
- space between atoms
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Dalton - Atomic Theory
- All matter is made of atoms that are indivisible and indestructible
- All atoms of a given element are identical in mass and properties
- Compounds are formed by a combination of two ro more different kinds of atoms
- A chemical reaction is a rearrangement of atoms
- Atoms cannot be created or destroyed
- EXPERIMENT: evaporation of water
- wondered how water and air could occupy the same space at the same time
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Crooke - Subatomic Particles (electrons)
- Crooke's Tube
- sealed glass tube with no air
- two electrodes
- high voltage between electrodes = electrons emitted from cathode
- electrons = cathode rays
- miss the anode and and hit the glass wall creating light
- behavior showed that they travel in straight lines
- Crooke's Tube
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Thomson - Charge/Mass Ratio of the electron
- Measured the mass/charge ratio of the cathode rays by measuring how much they were deflected by a magnetic field
- Electric and magnetic fields --> showed how he could balance the defelectiosn and obtain a straight beam
- Determined the charge-to-mass ratio of the electron
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Thomson - Plum Pudding moden of the atom
- atom is a solid and massive thing of positive charge dotted with negative charges
- electrons scattered throughout the atom
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Thomson - Charge/Mass Ratio of positive ions
- could only produce a single line on his photographs
- from its position he calculated it was te hydrogen ion
- mass to charge ratio was two thousand times greater than the electron
- to make sure --> used a large discharge tube at very low pressure
- managed to produce a second line, with twice the specific charge of the hydrogen ion
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Rutherford - Gold foil experiment
- two associates: Geiger and Marsden
- aimed a beam of alpha particles at a piece of gold foil that was about 8.6x10^-6 cm thick
- according to Thomson's model of an atom, the alpha particles should have passed directly through the gold foil
- most of the alpha particles went through, but some deviated
- 1/20,000 particles woudl deflect 90 degrees
- conclusion: an atom was more than just empty space and scattered electrons, an atom must have a postively charged center = most of mass
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Millikan - Oil Drop Experiment
- balance the downward gravitational force and the upward electrical and buoyant forces of charged oil droplets suspended between two metal plates
- mass and density of droplets known --> gravitational and buoyant forces could be calculated
- electric field was known --> charge on the oil drops could be determined
- their value of the charge of an electron = 1.5924x10^-19 C
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Mosley - Atomic number
- periodic table was ordered by atomic masses
- Moseley's experiment in X-ray spectroscopy showed that cobalt and nickel have different atomic numbers
- found a linear relationship between the atomic number and a measureable property of the nucleus
- showed that there were gaps in the atomic number sequence
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Chadwick - Discovery of the neutron
- other scientists thought there was something with protons in the nucleus - called it the neutron
- Chadwick repeated experiments of Frederic and Joliot-Curie
- looked for something with the same mass as a proton but with zero charge
