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400 BCE
Early Ideas: Democritus
With Leucippus, they had ideas that the atom was the building block of all matter. Physical properties were differentiated by shape/ arrangment of each atom. However, the idea was discredited by Aristotle which influenced the delay of more knowledge until 2000 years later -
John Dalton's Theory
A modest man of quaker background had four compenents to his theory: 1. All matter is made up of atoms and they are indestructible and invisble
2. All atoms of an element have the same mass and properties
3. Compunds are formed by a combo of two or more different kinds of atoms
4. A chemical reaction is a rearrangement of atoms -
J.J. Thomson and electrons
J.J.Thomson claimed that “Deflection of rays by electrically charged plates” were evidence enough to proof that there were bodies much smaller than atoms -
J.J. Thomson's Atomic Theory
Thomson suggested a "plum pudding" theory where an atom was simply a spherical positive charge with electrons scattered around like raisins in a plum pudding -
New Ideas: Rutheford and Bohr
One of the most well known experiments to contribute to the knowledge of atoms, where alpha particles were shot at a piece of gold foil. While this would be expected if atoms were in a pudding model, some bounced back. Rutherford concluded that an atom must have most of its mass as well as a positive charge in the nucleus around 10,000 smaller than the atom itself. -
Modifications: Bohr
Bohr claimed that electrons existed in set orbits around the nucleus and that electrons jump between energy levels He also said that each shell could only hold up to a certain amount of electrons. -
New Discoveries: Chadiwick / Rutherford and the Neutron
When Chadwick and Ruthford were studying atomic disintergration, they kept noticing that the atomic number (number of protons) was less than the atomic mass. Because electrons have almost no mass, they knew that it had to be something else now known as the neutron which is in the nucleus
