John Dalton proposed Dalton's Atomic Theory. It may be summarised as
All matter is made up of very small particles called atoms
All atoms are indivisble. They cannot be broken down into simpler particles
William Crookes
William Crookes carried out an experiment to investigate what happens when an electron current is passed through a glass tube containing low air pressure.
Experiment 1
Crookes used a vacuum tube to pass the electric current through the air.
He found that radiation was coming from the cathode (negatively charged)
He showed the existence of this radiation by placing a Maltese cross inside the tube. He found that a shadow of the cross was formed at the other end of the tube.
Crookes called them cathode rays to describe the rays coming from the cathode
Cathode rays are invisble but glow when they strike glass
Experiment 2
Crookes placed a light paddle wheel, mounted on rails, in front of the cathode, in the vacuum tube
When he current was switched on the paddle wheel moved away from the cathode
Crookes concluded the vanes were being struck by particles coming from the cathode
Crookes deduced from his experiments that:
Cathode rays travel in straigh lines
Cathode rays cause glass to fluoresce when they strike it
Cathode rays possess enough energy to move a paddle wheel
J.J. Thomson
J.J. Thomson devised an experiment to investigate if cathode consisted of charged particles.
Experiment 1
Obtained a narrow beam of cathode rays by passing them through a small hole in the anode
The narrow beam passed between two metal plates halfway along the tube and then struck a fluorescent screen, causing it to glow
He found that id there was no charge on the plates the beam passed straight through the tube
Upon placing a positive on the top plate, the position of the spot moved upwards
Thomson concluded that they consisted of negatively charged particles
These negatively charged particles were called electrons
Experiment 2
He placed a large electromagnet outside the glass and adjusted the strength of the magnetic field
He found that electrons were also deflected in the magentic field. Using the magnetic field, he was able to bring the beam of electrons back to its original position
From his experiment, he was able to calculate the ratio of the charge of the electron to its mass
Charge of Electron/Mass of Electron = 1.76x10^11 coulombs per kg
Robert Millikan
He set up an experiment to determine the size of the charge on the electron
Experiment
He spreayed tiny droplets of oil between two metal plates
Using X-rays, he ionised the air between the two plates
As the oil droplets fell through the air they picked up electrons, becoming negatively charged
From various measurements taken, he was able to calculate the size of the charge of an electron as 1.6x10^-19
Millikan substituted this value into the e/m value
1.6x10^-19/Mass of Electron = 1.76x10^11
From this equation, Millikan calculated th mass of the electron as 9.1x10^-31 kg.
Ernest Rutherford
He studied the scattering of particles by a very thin sheet of gold
Experiment 1
He bombarded a thin piece of gold foil with alpha particles
Alpha particles are postitively charged
He used a phosphorescent screen made of zince sulfide to detect the alpha particles. A small flash of light appeared on the screen when struck by alpha particles
He found that some alpha particles were deflected at large angles as the passed through the foil
Most particles passed straight throught the gold foil with little to no deflection
A few particles were reflected back along their own path
Observation
Conclusion
Most alpha particles pass through the gold foil with little to no deflection
Most of the atom is empty space
Some alpha particles are deflected at large angles
The alpha particles are repelled when they pass near the small positive nucleus
A small number of alpha particles are reflected back along their own paths
A small number of alpha particles collide head on with the nucleus
Experiment 2
Rutherford continued bombarding various substances with alpha particles
He found that with lighter atoms succh as oxygen and nitrogen, small positively charged particles were given off as a result of the bombardment
They explained their results by saying that the alpha particles were breaking up the nuclei of the lighter atoms to realeas positively charged particles
These positively charged paricles were named protons
James Chadwick
James Chadwick discovered the neutron
Experiment
He bombarded a sample of beryllium with alpha particles
He discovered some type of radiation consisting of particles with no charge came from the beryllium
These neutral particles were penetrating enough to knock protons out of the paraffin wax
He concludede that the alpa particles were knockung these neutral particles out of the nucleus of the beryllium atom
He found that these particles had the same mass as the proton
