Atomic Theory


Atomic Theory Essay, Research Paper

In ancient Greek the word atom meant the smallest indivisible particle that could be conceived. The atom was thought of as indestructible; in fact, the Greek word for atom means “not divisible.” Knowledge about the size and make up of the atom grew very slowly as scientific theory progressed. What we know/theorize about the atom now began with a core theory devised by Democrotus, a Greek philosopher who proposed that matter consisted of various types of tiny discrete particles and that the properties of matter were

determined by the properties of these particles. This core theory was then modified and altered over years by Dalton, Thompson, Rutherford, Bhor, and Chadwick. The atoms original structure was simple, but as more and more research was done the atom became more complex and puzzling.

Our first benefactor of atomic theory was John Dalton, a man later nick-named the “Father of atomic theory” for his contribution of many theories and laws to modern atomic theory. His theories answered many questions of skeptical scientists: elements combine with one another to form chemical compounds and the atom doesn’t change, atoms link together in definite proportions, all atoms of any element are all the same, the law of multiple proportions which states that a given mass of one element can combine with various masses of another element (or elements) but always in small whole number ratios, and the law of conservation of matter, matter can neither be created or destroyed, but it can change form.

One of the next most recognized people involved in the progression of atomic theory was J.J. Thompson. Thompson had researched the work of William Crookes whose research concluded that cathode rays were deflected by magnetic fields. Thompson elaborated on this conclusion and found that cathode rays were also deflected by an electric field. With much experimentation Thompson theorized that although the atom was made up of small particles it was not the same indestructible model proposed by Dalton. A man named Milikin determined the mass of an electron to be 0 amu and the relative charge to be negative 1. With the discovery of these electrons by Thompson and the discovery of protons several years later Thompson was able to devise a new model of the atom. Thompson stated that protons and electrons were evenly distributed throughout the atom. This model was labeled the “plum-pudding model”. Thompson described the atom as a gooey mass of positively charged particles (protons) with raisins (electrons) embedded all around it.

Ernest Rutherford was a major contributor to the atomic model because he proved that although Dalton and Thompson were partially correct there was still a flaw in both of their models of the atom. Rutherford devised an experiment to either prove or disprove Dalton’s model and or Thompson’s model. Rutherford and his assistants put a piece of radioactive material in a lead box with a small hole in one side to direct the alpha particles towards the gold foil, which was surrounded by zinc sulfide screens. The zinc screens showed flashes of light where the alpha particles were going to go, straight through (Thompson’s model) or straight back (Dalton’s model). To his astonishment particles were through, came back, and some also were deflected to the sides of the gold foil. He proved that Dalton and Thompson were incorrect and the atom consisted mostly of space. At the center of this space is a very small core, called the nucleus, which can justify the deflected particles. Rutherford established that the mass of the atom is concentrated in its nucleus. He found that an electron is 1/1836 the mass of a proton and he also proposed that electrons travel in orbits around the nucleus.

With all of these alterations to the theory of an atom a few, five to be exact, problems arose. One of the major problems was the size of an atom. If each electron had its own orbital and the atom had 23 electrons then the atom would be enormous. Another problem with the orbital of an electron was that no energy could be observed by the electron orbit decay. Next, if the center of an atom was composed of protons (+) and the electrons (-) orbited this positive core why didn’t the electrons crash into the protons, causing an ultra violet catastrophe. Also, if the core was composed of just positive protons and opposite charges repel then how did the protons stay together. And the final problem, the atom didn’t weigh enough. When scientists added the weight of the electrons and the weight of the protons and subtracted that from the overall weight of the atom there was a remainder. Something had to be missing from the model of an atom to make up for the weight difference. The answers to these questions came from the next big contributors to this theory, Niles Bohr and Chadwick.

Niles Bohr developed a theory known as the Bohr theory of the atom. He assumed that electrons are arranged in definite energy levels, or quantum levels, at a specific distance from the nucleus. The arrangement of these electrons is called the electron configuration. There are seven levels, which were derived from the seven colors he saw, each of which has a specific number of electrons that it has capacity for. The first level can only accommodate two electrons, the second can hold up to eight electrons, the third can hold up to eight-teen, and so on. If an atom had four electrons you wouldn’t find two in the first, one in the second, and one in the sixth. Electrons always occupy the lowest energy levels first. Electrons in a “ground state” are in their regular energy level and give off no energy; however, if an electron is in an “excited state” it sends energy in quantum packets (photons) and light is observes. When excited electrons jump up a level they give off light energy: however, they can never go down a level, energy can never be lost only gained.

Chadwick discovered the last piece of information about the atom. In 1932, through mass spectronomy, he discovered a heavy neutral particle with the mass of a proton. He called it a neutron. This answered the few remaining question about the atom. There was no longer a missing part because the neutron made up for the original weight loss. And it answered the question about how the protons stayed together. The neutrons had a soothing effect on the protons. Between every two protons lies a neutron, which attracted the protons, which made the nucleus very condensed.

With the last discovery, of the neutron, scientists were able to complete the model of the atom. The completed model that they came up with is the model that students now learn about in school. These scientists did exactly what scientists are supposed to do: test, experiment, and answer questions. Because of the years of study they did we now have a strong idea of what an atom is and what its components are. A theory never becomes fact until all of the bugs are wiped out, if this is true then this atomic theory is well on its way to becoming the facts about atoms.



Textbook: Chemistry: The Study Of Matter, Simon and Schuster, copyright 1992

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