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The Lives of Those Who Made Contributions to the Atomic Theory

A slightly detailed description of some of the people who discovered or fixed and added bits and pieces to today's atomic theory.
by

Alexa Neithercut

on 16 March 2011

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Transcript of The Lives of Those Who Made Contributions to the Atomic Theory

Democritus was a Greek Philosopher who was among the first to suggest the existence of the atom. All living things are composed of tiny particles called "Atoms". Atoms are made up of a cloud of electrons surrounding a nucleus filled with protons and neutrons. His ideas agreed with modern scientific theory, but back in his time they could not explain chemical behavior, nor did they have the scientific evidence to back them up. John Dalton, an English Chemist, discovered the real nature of atoms and the connection between observable changes and events at the atomic level 2000 years after Democritus. By experimenting, Dalton proved Democritus's ideas and transformed them into a scientific theory. Based on his experiments, he came up with 4 theories to explain his results; All elements have atoms. Atoms in the same element are identical. Atoms of different elements can combine physically, or chemically in simple whole-number ratios. Chemical reactions occur when atoms are seperated, joined, or rearranged. Atoms cannot, however, change between elements and will always retain their original code. The Lives of Those Who Made Contributions to the Atomic Theory By Alexa Neithercut J.J. Thomson, an English phsyicist, discovered the electron. The electron is a negatively charged particle residing outside the nucleus.
(Which had yet to be discovered) John Dalton discovered the real nagture of atoms and the connection between obseravble changes and events at the atomic level 2000 years after Democritus. In 1897, an English physicist named J.J. Thomson discovered the electron by experimenting with a cathode ray. In his experiments, he was using basically a magnet tube, with a positive and negative end. Since the cathode ray was always attracted to the cathode end, he theorized that the beam was filled with tiny negatively-charged particles. Robert Millikan, a U.S. Physicist, did experiments to find the charge of the electron.

Using the value and the charge-to-mass ratio measured by Thomson, he also discovered the mass. His findings are very similar to today's accepted measurements;

An electron carries exactly 1 negative charge and is 1/1840 the mass of a hydrogen atom. In 1886, another subatomic particle was discovered;
the proton. Using the same experiment that J.J. Thomson used, Eugene Goldstein discovered there was another beam traveling in the cathode ray;
a beam traveling the opposite direction of the cathode--straight to the anode. In 1932, James Chadwick discovered the last subatomic particle;
the neutron.

With a mass equal to the proton, they carry no charge. In 1911, a student of J.J. Thomson, Ernest Rutherford, conducted an experiement to test the current theory of atomic structure. In this experiment, they used alpha particles aimed at a sheet of gold foil. According to the accepted theory of that time, most of the particles should have deflected.
However, many of them passed through, astounding Rutherford and his scientists. From this experiement, Rutherford concluded that the atom must be filled with empty space.
The rest of the space, however, was composed of a tiny center where all the postively charged particles and mass were located.
He called this the nucleus. A young Danish physisict named Niels Bohr and a student of Ernest Rutherford believed his teacher's model needed improving. In 1913, he changed the atomic model to include new discoveries about how atomic energy changes and when it absorbs or emits light. He proposed that the atom has electrons that only travel in specific paths around the nucleus. In this model, he proposed that electrons can only move through fixed levels of orbit. To move levels, an electron must gain or lose energy. In 1932, Louis de Broglie suggested that moving particles like electrons have some properties of waves. Later on, experiemental evidence will support this idea. Erwin Schrodinger took the ideas developed by Broglie and put them together in an equation.

Solving this equation can predict the properties and reactivities of all atoms and molecules.

Antoine Lavoisier is known as the Father of Modern Chemistry. He discovered that water was made of hydrogen and oxygen. He also showed that chemical elements were neither created nor destroyed, just combined into different compounds in chemical reactions. From this, we get the conservation of mass.
Henry Moseley came up with the concept of the atomic number. He also sorted all of the elements into the "Periodic Table of Elements", a chart of the elements based on their physical order.

In the 1700's, Benjamin Franklin was the first to suggest that electricity was made up of positive and negative charges.

This was the forefront of John Dalton's four postulates on atoms.
Dalton viewed atoms as tiny balls. Thomson viewed atoms as mostly charged material, like dough in a bun. Rutherford believed in the nucleus as the center of the atom. Bohr believed that, in the atom, there were different rings outside of the nucleus that electrons would jump between. Chadwick discovered that neutrons were also a part of the atom. The modern atomic model consists of an electron cloud, where it is impossible to know where an electron is at any given time. ALSO!!!.........
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