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John Dalton
John Daltons atomic theory;
All matter is made of indivisible, indestructable atoms.
All atoms of the same element are identical in mass and properties.
Compounds are formed by a combination of different atoms,
A chemical reaction is a rearrangment of atoms.
He also created the first chart of atomic weights. -
J.J. Thomson
J.J. Thomson discovered the electron in 1897 in his cathode ray experiment.
He interpreted the deflection of rays by electrically charged plates and magnets as evidence of "bodies much smaller than atoms"
Later he estimated the value of the charge itself.
In 1904 his model of an atom was a sphere of positive matter which electrons are organized by electrostatic forces.
Plum pudding model -
Ernest Rutherford
Rutherford devised his gold foil experiment that forced him to come to the conclusion that Thomsons model was incorrect and the mass of an atom could not be equally distributed but concentrated in the middle of the atom.
Some of his main points include;
- A major part of the atom is empty
- Mass is concentrated in the nucleus
- Positively charged particles are in the nucleus
- Electrons revolve around the nucleus in different orbits
- Nucleus is very small compared to the atom -
Niels Bohr
Niels Bohr applied quantum theory to Rutherford's atomic model by assuming electrons travel in stationary orbits (such as planets in the solar system). The stationary orbit defined by angular momentum led to the calculation of possible levels of energy for these orbits. When an electron falls from an outer to inner orbit the color of light given off depends on the energy levels of the two orbits. -
James Chadwick
Found that the nucleus also included uncharged (neutral) particles. Credited with the discovery of the neutron. He reasoned neutrons were important in holding the protons together. -
Modern Model
Through recent research scientists have concluded that electrons do NOT travel in neat planet like orbits. They are actually sperical regions of space where electrons are most likely to be found. Today we know that we cannot depict the exact whereabouts of electrons, we can only describe the probable location of them with fuzzy boundaries (clouds).
