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Atomic Theory Research Paper

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Of all of the many scientists who played a major role in the formation of the atomic theory, it is believed that Dalton, Rutherford, Thompson, Bohr, and Planck are the five most influential. The research of this theory has gone on for many years and each and every one of them have brought us closer to our current understanding of the atom today. Each one of them played a special role in the development process of the atom and most importantly, they all reflected on one another. Many influential scientists follow up on one another proving their own theories correct and in time, create challenges for the next scientist to question. This truly, is one of the main reasons we have gotten so far with the atom. These amazing discoveries mostly started taking effect when Dalton came into the picture.

John Dalton (1766-1844), is one of the most influential scientists in the development of the atomic theory. Democritus first suggested the existence of the atom, but it took nearly two millennia before Dalton took the research into his own hands. Dalton’s Atomic Theory consists of basic concepts based on the theory that atoms of different elements could be distinguished by differences in their weights. In this he concluded that, 1. All atoms are composed of elements. 2. The atoms of a given element are different from those of any other element. 3. Atoms cannot be created, divided into smaller pieces, or destroyed in the chemical process. 4. Atoms of one element can combine with atoms of other elements to form chemical compounds. 5.

When elements react, their atoms sometimes combine in more than one simple, whole-number ratio (Senese, & De Leon). Dalton’s model was also that atoms were tiny, indivisible, indestructible particles. Also in his model, he states that each atom has a certain size, mass, and chemical behavior. Unfortunately, John Dalton made a mistake in rationalizing the many laws of chemical combination. He assumed that the “simplest compound of two elements must be binary, formed from atoms of each element in a 1:1 ratio” (De Leon). His atomic weight system ended up a little inaccurate and he gave oxygen an atomic weight of seven instead of eight. Despite these errors, Dalton’s theory remains to this day, the basic fundamentals of the atomic theory’s countless developments.

This next scientist made an atomic model called the Plum Pudding model. His name is J.J. Thompson and he created his model after the components of the atom were discovered in 1911. It was concluded that atoms were made up of subatomic particles called protons and electrons. Although that was discovered, there was no explanation as to how they were distributed among the atom. So, he demonstrated that cathode rays were units of electrical current made up of negatively charged particles. Thompson theorized his model of atomic structure in which negatively charged electrons were “embedded in a sphere of positive electricity, neutralizing each other” (Atomic Magic). Not only did Thompson win the Nobel Prize for his outstanding achievement, he also turned his attention towards positively charged ions. He discovered that neon gas was made up of a combination of two different ions.

He came to this conclusion by using magnetic and electric fields to deflect the positive ions of the gas onto two different parts of a photographic plate. Because of this experiment, Thompson clearly discovered the possibility that your average element might exist as isotopes. Right then and there, he made his clear statement that cathode rays were indeed, made of particles and these particles came from within the atoms of the electrodes themselves, meaning that atoms are in fact divisible. Because of this statement, Thompson proved Dalton wrong and came to his creation of the Plum Pudding model. As time goes on, these incredible scientists find themselves creating new and exciting questions for those to come.

Thompson’s efforts to estimate the number of electrons in an atom initiated the experiments of his student Ernest Rutherford. His conclusion of the atom is that it contains a positive nucleus surrounded by negative orbiting electrons. His model also stated that the atom’s mass was mostly contained in the nucleus, while the rest was mostly empty space. Rutherford came to this conclusion by his famous gold foil experiment. This experiment involved firing of radioactive particles through thin, gold, metal foils and detecting them using a scintillator (screens covered with zinc sulfide). He then found that the majority of the particles passed right through the foil and only about 1 out of 8000 were deflected (Chemsoc Timeline). This experiment led him to the theory that most of the atom was made up of empty space. Rutherford’s model of the nuclear atom proved Thompson’s model wrong by stating that an atom consists of an electrically charged nucleus surrounded by alpha particles called electrons.

He then went even further and sought out a well thought out explanation proving his scattering of the electrons. This is called Rutherford’s Scattering Formula (Microscopic World Part 2). Rutherford assumed that the positive energy concentrated in the nucleus and the alpha particles (electrons) are scattered with a repulsive Coulomb force. Basically, if the incident line is close to the nucleus, the trajectory of the electrons would be widely curved and would scatter. Rutherford’s formula fit very well to the model and thus, his model of the nuclear atom became established in the modern world of science.

Because of Rutherford’s model, a scientist by the name of Neils Bohr elaborated on the Rutherford Model. Many people believe that Rutherford’s Atomic Theory is essentially the same as Bohr’s Atomic model, but that is untrue. Yes Bohr’s model was inspired by Rutherford’s, but he built upon it. The main difference is the fact that Bohr included a thesis for the forces that hold the atomic structure together. Bohr’s model went further in depth to say that the electrons orbit the nucleus. He then created quantum physics, because only a certain number of electrons can occupy each orbital.

He assigned a quantum number to each orbital, considering the orbitals closest to the nucleus are assigned the lowest number (Titov, Feldmen). Electrons occupy the lowest energy levels closest to the nucleus first because they are “lazy”. He also described that electrons jumping to a lower state gives off its energy in the form of light. This then, was how he described the quantum leap. This theory raised a lot of questions as to why the electrons can’t exist in between quantum states. It is still unanswerable, but true of the entire quantum world. Bohr’s model is a more primitive model of the Hydrogen atom and is very basic, but still a very important part of the later discoveries.

In 1932, James Chadwick made a very important discovery. He found that the atom is not just made up of protons and electrons, but of particles called neutrons as well. He performed tests on new types of radiation which have mistakenly been called gamma rays for years. He first took a sample of Beryllium bombarded by alpha particles which cause it to create the radiation. Chadwick began to fit all the pieces of the puzzle together by noting that the “mysterious radiation was neutral due to the fact that it was not affected by proximity to a magnetic field, and, unlike standard gamma radiation, did not invoke the photoelectric effect” (McPhee). Instead, it discharged the protons, meaning that the particles had to be larger than expected. Chadwick then created an equation for solving the mass energy of the neutron. After his experiment, he used the equation to get a neutron mass of 938 +/- 1.8 MeV. Because of Chadwick, the atom is now more complete then ever and fundamentally correct.

Through all the many scientists who helped discover the atom, I believe the above five are the most influential. John Dalton is the number one because he paved the ay for further research. He used all the knowledge he had and created the basic fundamentals of the atom. Even though Democritus was the first to question the atom, Dalton was the first to experiment and take matters into his own hands. I believe the second most influential would be Neils Bohr because he used quantum physics to explain Rutherford’s model. So in that sense I believe Ernest Rutherford is number three. They both were an extremely important part of the atomic model because they both proved J.J. Thompson’s model completely wrong.

Rutherford proved that an atom contains a nucleus and protons and electrons, while Bohr proved his model by using quantum physics. Then number four is James Chadwick because he proved that atoms contain neutrons by using his radiation experiments. This was an important discovery in a sense that he proved to Bohr and Rutherford that an atom does not only include protons and electrons. J. J. Thompson is number five in my book because although he was very important, his model was extremely off and he did not have as much evidence to back up his theory. So once again, all five of these famous scientists affected one another creating a powerful chain reaction of experimentation to create the powerful Atomic Theory. I do believe that there is more to come of this. With all of these new discoveries in technology, there’s no telling what will come next. Just when Rutherford and Bohr thought that the atom was complete, Chadwick came and added the neutron. I believe this could happen and we could find something more exciting about the atom, or even what its incredible powers can do.


1.De Leon, Professor N. “Dalton’s Atomic Theory.” Chemistry 101 Class Notes. 14 Feb 2009 .

2.Senese, Fred. “Foundations of Dalton’s atomic theory.” General Chemistry Online. 25 July 2005. 14 Feb 2009 .

3.”Rutherford-Atomic Theory.” Chemsoc Timeline. 15 Feb 2009 .

4.”Atomic Magic.” J.J. Thompson. Atomic Magic. 15 Feb 2009 .

5.”2-5 Rutherford’s Atomic Model.” Internet Seminar Microscopic World Part 2. 15 Feb 2009 .

6.Titov, Feldmen, Sergei, Vladislav. “The Atomic Theory.” Think Quest. 15 Feb 2009 .

7.McPhee, Isaac M.. “The Discovery of the Neutron.” Suite101.com. 27 Feb. 2008. 15 Feb 2009 .

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