Notes

- Democritus was an ancient Greek philosopher who lived from 460 BC to 370 BC. He was an influential philosopher who taught many things that were influential to people today. Democritus claimed that everything is made up of atoms. These atoms are physically, but not geometrically, indivisible. He believed that in between atoms is empty space. Atoms, he philosophized, are indestructible when in motion.

- John Dalton provided several ideas based on his atomic theory. He stated,
1. Matter is made up of extremely small particles called atoms which retain their individuality in all chemical transformations.
2. Atoms of the same element resemble one another in all respects and have different weights.
3. Atoms of different elements have different weights and posses different properties.
4. Atoms of different elements combine in simple but fixed ratio to form compound atoms.
5. Atoms are indestructible.
Dalton's Atomic Theory:
1 oxygen atom at 16 mass units each (16 mass units) + 2 hydrogen atoms at 1 mass units each (2 mass units) = 1 water molecule at 18 mass units each (18 mass units)
* All elements are composed of atoms (indivisible particles)
* All atoms of the same element are identical - in particular, they have the same mass.
* The atoms of one kind elements are different from the atoms of all other elements- in particular the atoms of one element have a different mass than those of other elements.
* Atoms are indestructible and retain their identity in chemical reactions.
*Compounds are formed by joining atoms in specific whole number ratios.

- In 1897, J.J. Thomson discovered the electron. He found out by doing experiments on a cathode test tube to study the nature of electric discharges. In 1904, Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces.

- In 1898, Ernest Rutherford developed his atomic theory that was significant to the atom itself. His theory was that the atomic structure that varied significantly from past theories on the same subject matter. Later in 1911, he makes another contribution to the atom. He believed that the atom had a central positive nucleus. This was another great idea made during his time.

-Hantaro Nagaoka, this man made a quite significant theory on the atom. He believed that electrons revolve around the positively charged center of the atom like the rings of Saturn. Although, other scientists, such as Rutherford and Dalton have proved him wrong.

-Robert A. Millikan determined that the unit charge of the electron in 1909 with his oil drop experiment at the University of Chicago. Thus allowing for the calculation of the mass of the electron and the positively charged atoms. He proved it by doing his oil drop experiment and his photoelectric effect experiment. Robert used his oil drop experiment to discover the electron's charge independently.

- Mosely discovered that the energy of x-rays emitted by the elements increased in a linear fashion with each successive element in the periodic table. In 1913, he proposed that the relationship was a function of the positive charge on the nucleus. This rearranged the periodic table by using the atomic number instead of atomic mass to represent the progression of elements. This new table left additional holes for elements that would soon be discovered. Unfortunately, Moseley was killed at the Gallipoli during WWI. He proved his theory by providing experimental evidence to support Niels Bohr's theory.

- Mr. Bohr proposed quite a significant theory on the atom. He proposed that electrons orbit nucleus in defined energy levels. He did this by experimenting how much radiation the positive electrons emit.

-Louis de Broglie, his ideas were a basis for developing the wave mechanics theory. This theory has greatly improved our knowledge of the physical nature on the atomic scale. Although, he was not able to prove this theory.

- Werner Heinsberg discovered the theory of the Quantum Mechanics. Quantum Mechanics, according to wikipedia.com, is a branch of physics which deals with physical phenomena at nanoscopic scales where the action is on the order of the Planck constant. It departs from classical mechanics primarily at the quantum realm of atomic and subatomic length scales.

- Erwin Schrodinger used mathematical equations to describe the location and energy level of an electron. He developed the quantum mechanics model (the widely accepted atomic model today). He also proposed the wave theory of matter. He proved it by, again, using mathematics towards his theories.

- James Chadwick proved the existence of neutrons in 1932, according to Book Rags. As uncharged particles in atoms, neutrons play a key part in nuclear fission, or the splitting of atoms. He also used his work on the atom to devise uranium 235, a main component in the atomic bomb.

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