Perkins.Singleton.Timeline.Fall.2011


 * Historiam Atom ||
 * An account of the Atom’s change in perception through the ages. Starting from its conception in Ancient Greek Philosophy and ending at its current and future stages of comprehension in the modern sciences. ||
 * **Nicholas Perkins and Michael Singleton** ||
 * **12/5/2011** ||
 * **Nicholas Perkins and Michael Singleton** ||
 * **12/5/2011** ||

The history of the Atom is an extensive and interesting one. Throughout ancient and modern times it has always been a topic of mystery and debate due to the astounding unknowns that encase it. This lead to the creation of numerous postulations of its identity and purpose in the universe, which were to be readjusted, and eventually replaced as knowledge about this entity came forth through the advancement of scientific knowledge allowing for the formulation of advanced hypotheses and experiments to come about through the ages. Such an astounding existence could only come from a beginning of equal awe. In the cradle of civilization, Ancient Greece and the Orient, the conception of the Atom was to take place, not by a scientist but by a philosopher, who was to start the journey that would begin to answer the age old question of what are substances comprised of.

In the fifth century BCE the two philosophers, Leucippus and his pupil Democritus, conceived and perfected the concept of the //atomos//, or as would be recognized by later generations as the atom. Their goal was not of science, but of philosophy, to discern the entity that comprises all of existence and its properties to assist in explaining the natural world. Leucippus and Democritus conjectured that

…the two fundamental and oppositely characterized constituents of the natural world are indivisible bodies—atoms—and void. The latter is described simply as nothing, or the negation of body. Atoms are by their nature intrinsically unchangeable; they can only move about in the void and combine into different clusters. Since the atoms are separated by void, they cannot fuse, but must rather bounce off one another when they collide. Because all macroscopic objects are in fact combinations of atoms, everything in the macroscopic world is subject to change, as their constituent atoms shift or move away. Thus, while the atoms themselves persist through all time, everything in the world of our experience is transitory and subject to dissolution.[i]

Though aspects of this original atomic theory were correct, such as how the chemical makeup of an atom determines the outcome of matter and that atoms have the ability to combine with one another, it was astronomically flawed and vastly remote from the truth. To understand then the need for such a principle, one must go back further than Leucippus, to two other ancient Greek philosophers, Heraclitus and Parmenides. In the sixth century BCE the philosopher Heraclitus stated that all things in the natural world are subject to change and are so never permanent or fixed like the movement of water in a river.[ii] His detractor though, was Parmenides, who in turn made the rebuttal that all things in the natural world are not subject to change, but instead

…everything is what it is, so that it cannot become what it is not (change is impossible because a substance would have to transition through nothing to become something else, which is a logical contradiction). Thus, change is incompatible with being so that only the permanent aspects of the Universe could be considered real.[iii]

This debate generated much interest and speculation of what comprises the natural world, and allows substances to form into their present states. Also, the greater question was who was correct, are substances in the natural world subject to change, or are they fixed entities that exist for all time. Many years would pass until a solution would arise, bringing us back to Leucippus. Leucippus, a natural philosopher, developed the atom as a solution to this debate. His theory would create unison between the two factions by stating that both theories were correct in their own respects, by declaring that change does exist but can only come to pass if specific atoms combine in the correct reaction, and if this does not occur change does not take place at all, everything remains as it were; placing the atom, for a miniscule period of time, in the role as the deciding factor of creation and prohibition of change in substances. The atom though, soon became discredited by two of the greatest philosophers of all time, Aristotle and Plato. Both learned men agreed that it was absurd …to reduce the whole of reality, including human beings, to a system that knew nothing but moving atoms. Even with respect to the problems of the material world, atomism seemed to offer no sufficient explanation. It did not explain the observable fact that, notwithstanding continual changes, a total order of specific forms continued to exist. For this reason Aristotle, with Plato, was more interested in the principle order than in that of the material elements. In his own analysis of change, which resulted in the matter-form doctrine, Aristotle explicitly rejected the thesis of Democritus that in a chemical reaction the component parts retain their identity.[iv]

Stating to the people that there was no sound reasoning behind the theory, that atoms if they even existed played no part in the natural world, due to the inconsistencies in the explanation of natural phenomena and how it was derived not from a world of ideals or logical analysis known to man, but from conjecture concerning an impossible aspect of the universe. Demonstrating how it was inconceivable to people that such a miniscule entity could control nature and do so with its own laws that differ dramatically from our own perception of the universe. Causing the development of the idea of the four elements, which represent the ancient view of existence that things are what they seem to be unless dealt with in idealistic philosophical terms. With such limitations of people’s perception and detractors such as Aristotle and Plato proffering ideas of logical analysis of the cosmos through ideals and facts, the atom fell from grace and would lay dormant for millennia.

In Medieval Times and again in the Renaissance, the atom was reexamined by scientists in Europe and in Islamic domains, and through their research the atom regained credibility as an important and factual aspect of nature. During this era of history in the Orient, second century A.D. to the Renaissance, science had started to develop its own separate identity from philosophy and so became more scientific and critical and less idealistic in nature. In Europe though “science was (integrated with) philosophy, and theology.”[v] This integration would cause European science to be not as advanced as its counterpart in the East, but eventually the harmony between science and religion would cease as new theories were introduced into Europe from expanding relations with the Orient. Until that time though, European science remained in a primitive form. In Europe the form of science that examined the atom the most definitively was alchemy, the other areas of study utilized the extremely scarce ancient texts as tools to explain natural phenomena and religion. Even though alchemists were not researching atoms directly, their quest to morph substances into new and different configurations and understand the composition of substances involved the basic fundamentals of atomic manipulation. Alchemy’s purpose was not just to discover the ever elusive formula that would create the //Philosopher’s Stone//; it also served a more practical purpose for its day and age. The research of the alchemists

…occasioned an intense practical involvement with minerals, metals, and the making of medicines. Alchemical procedures produced effects and led to the analysis of various parts of the natural world…(showing) that alchemy was a rational subject, did have utilitarian value, did develop according to prescribed procedures, and could be taught…Even when…procedures and projects lacked success, the involvement with alchemical…processes…had implications for further knowledge...[vi]

Through such ingenious endeavors knowledge of chemical reactions and other scientific facts were gained, but there was still a prevalent chasm of maturity between European and Arabic science. Ever since the fall of the Western Roman Empire, the barbarity of the ruling Germanic Tribes overrunning Europe, and the Church becoming totalitarian in society, Europeans had become separated from Classical thoughts and knowledge unlike their Byzantine counterparts. In the Orient, science was rapidly advancing due to how

The torch of ancient learning passed…in the 7th century (to)…the Arabs…To the Arabs; ancient science was a precious treasure. The [|Quran], the sacred book of Islam, particularly praised medicine as an art close to God. Astronomy and astrology were believed to be one way of glimpsing what God willed for mankind. Contact with Hindu mathematics and the requirements of astronomy stimulated the study of numbers and of geometry. The writings of the Hellenes were, therefore, eagerly sought and translated, and thus much of the science of antiquity passed into Islamic culture. Greek medicine, Greek astronomy and astrology, and Greek mathematics, together with the great philosophical works of Plato and, particularly, Aristotle, were assimilated in Islam by the end of the 9th century. Nor did the Arabs stop with assimilation. They criticized and they innovated…[vii]

One of those areas of modification was the atom. In the East the theory of the minima naturalia, which was an ancient Greek theory of the atom created after Aristotle’s time that stated that “…atoms were called //elachista// (“Very small” or “smallest)… (And)…Each substance had its own minimum of magnitude below which it could not exist… (or if) divided…would become a minimum of another substance.”[viii] This doctrine was favored by Arab scientists known as Averroists, who applied it to their experiments to understand the world. It was not until the Renaissance when two Italian scientists, Agostino Nifo and Julius Caesar Scaliger, brought this theory into its glory. By that time Europe and the Orient had become joined in a beneficial symbiotic relationship that supported the exchange of ideas and knowledge among other commodities. This unison mainly came from how

Medieval Christendom confronted Islam…in military crusades, in Spain and the [|Holy Land]...From this confrontation came the restoration of ancient learning to the West. The Reconquista in Spain gradually pushed the Moors south from the Pyrenees and among the treasures left behind were Arabic translations of Greek works of science and philosophy. In 1085 the city of [|Toledo], with one of the finest libraries in Islam, fell to the Christians. Among the occupiers were Christian monks who quickly began the process of translating ancient works into Latin. By the end of the 12th century much of the ancient heritage was again available to the Latin West.[ix]

This reconnection with ancient texts would propel Europe toward scientific enlightenment and maturity. Paving the way in atomic chemistry was Agostino Nifo, who would inspire later scientists to further infer answers to the mysteries of the atom. Agostino Nifo, along with other scientists, still held the ancient Greek conception of the image of an atom, but through experimentation and observation was able to prove beyond a reasonable doubt that atoms do in fact exist and so affect

…growth, generation, and alteration…that there are maximum and minimum degrees of any naturally intensible form…The agent can alter the first minimum part of the subject by one degree of quality, then by means of the first it will alter the second to one degree; while it alters the second to one degree, it will push the first up to two degrees; [etc.]…By flux should be understood the reception by which the subject successively receives the form…[x]

Describing the creation of compounds and molecules by how atoms from different atoms affect one another during a reaction. This discovery singlehandedly disproved Aristotelian theory of atoms not existing and so atoms do not affect any aspect of nature or the divine. Also, he further separated science from religion by expelling atoms from any aspect of religion and having them recognized as an actual component of chemistry. Through this astronomical breakthrough, atomic chemistry was born, and the man who was to continue to lay its foundation was Julius Caesar Scaliger. Julius Caesar Scaliger expanded Agostino Nifo’s theory by incorporating the infant rules of atomic theory and their effect on mixtures and compounds through his published research which stated that

The nature of the elements [is understood] not only with the respect to themselves but also with respect to their mixts. With respect to itself, it [each element] has a form which it gives up in order to obtain a nobler form [in the mixt]. Thus neither do the forms [of the elements combined in the mixt] remain, nor are the qualities deprived of their forms, but in a different way they are accommodated to the substance of the mixt. For a new generation it is necessary that the forms of the parts, subdued by one another’s qualities, should have laid aside the original inflexibility of nature under the dominion of one [form] that is more powerful.[xi]

Describing how then unknown process of how electrons are given up to form compounds (ionic), that the atoms remain the same in a compound but are rearranged in appearance, and that a stronger bond will overpower a weaker one in a reaction. Julius Caesar Scaliger went on to state in his theory that a continuous body is not created by atoms but what did occur would not be a mixture but “a heap – as in the atomic theory… (Believing)…that the distinctive property…is the continuity of its constituent parts”[xii] initializing the understanding of molecules combining to create mixtures. His final theory on the atom concerned the composition of an atom, in which the idea of protons, neutrons, and electrons was briefly touched upon, and that reactions can generate byproducts. Julius Caesar Scaliger conclusions in his work //Exotericarum Exercitationum Libri XV de Subtilitat,// were

…the importance he gave to the motion, size, and arrangement of minima (atoms). He believed that minima (atoms) of different substances, including the four elements, differ in size…The arrangement of minima (atoms) explains the different states of aggregation of bodies…also… (Byproducts are)…the cause of some chemical reactions, for example the production of heat when quicklime is mixed with water...[xiii]

Even though his description of the nuclear process that was taking place was incorrect, he correctly identified that how an atom’s composition of what is now known as subatomic particles affect what a substance will be, and he successively determined that chemical reactions create byproducts. From the publication and verification of these revolutionary theories the atom had a bright future of further advancement in the approaching Scientific Revolution, for no longer was it viewed as an intangible philosophical ideal but an actual component of legitimate science.



Julius Caesar Scaliger's treatise on his contribution to the atom. Source: http://www.jnorman.com/hss/images/items/7724.jpg

In the eighteenth century the atom, along with chemistry in general greatly progressed. A select few of the great scientists who contributed to this advancement were Lavoisier, Richter, and Proust. These gentlemen through their harrowing experiments laid the foundation for modern chemistry and opened the gates that interred the answers to the atom’s secrets. They lived during the great second renaissance, the Age of Enlightenment and the Scientific Revolution. This second renaissance liberated the western world once and for all from the ideology of the Middle-Ages, and brought the west into the dawn of a new age in which sound logical reason and the wonders of scientific advancement bettered mankind by improving daily life.

Antoine-Laurent de Lavoisier, the father of modern chemistry, lived from 1743 to 1794 when his life was unfortunately ended prematurely by the // Reign of Terror //. He singlehandedly revised chemistry and brought it out of the mysticism of the past into the logical present. Lavoisier did this first in 1777 by disproving the phlogiston theory, which erroneously stated that an invisible substance entitled phlogiston was emitted by substances and objects when they were burned. This theory was disproved when Lavoisier identified and explained the process of combustion through his experiments with respiration and oxygen, which showed that respiration, is a form of combustion and that oxygen is a participant in combustions. Next, he pioneered stoichiometry by the creation of his law of conservation of mass. Lavoisier performed quantitative chemical experiments that recorded every aspect of the test subjects in every step of the experiment. This exact recording of information revealed that the mass at the beginning of an experiment is the same at the experiment’s conclusion, surmising then that a balance must exist in the transformation between products and reactants. Finally, Lavoisier developed the use of chemical nomenclature in 1787 and composed a treatise of modern factual chemistry in 1789. Lavoisier through his research of compounds and elements realized the importance of a systematic naming system being needed to organize research. So in his paper //Méthode de nomenclature chimique//, he described how substances should be named by their compositions. In his treatise [|//Traité élémentaire de chimie//], Lavoisier put to rest the absurd archaic notions of chemistry as being entirely useless in modern chemistry, and summarized the new found knowledge in chemistry and its application and purposes.[xiv]



Jeremias Benjamin Richter, chemist to the royal porcelain factory at [|Berlin], lived from 1762 to 1807. His contribution to chemistry was titration. He developed the concept of titration through his work with acids and basses. Through his work he discovered how much acid is needed per part to saturate a base solution and vice versa, and also that the amount of bases needed to saturate an acidic solution are equivalent to one another.[xv]



Joseph Louis Proust, a professor of chemistry at [|Segovia] and at the [|University of Salamanca], lived from 1754 to 1826. His significant contribution to chemistry would provide evidence that atoms do in fact exist and push science towards formulating an atomic theory. His monumental discovery was the law of definite proportions, also known as Proust’s Law. In 1794 Proust published his treatise that documented his discovery. His discovery came from his experiment with artificial copper carbonate and natural carbonate; in it he compared the two substances and proved that there was a proportion of mass between the three elements and that and that no other compounds existed between his two test compounds. Resulting in the law stating that the ratio by weight of the compounds consumed in a chemical reaction is always the same. However, non-stoichiometric compounds do not follow this law because of the fluctuation of the amount of elements that occurs, causing the ratio to change.[xvi]



[i] Berryman, Sylvia, "Ancient Atomism", //The Stanford Encyclopedia of Philosophy (Fall 2008 Edition)//, Edward N. Zalta (ed.), URL = . (Accessed Tuesday, September 27, 2011) [ii] Venable, F.P., Ph.D., D.Sc., LL.D. //The Study Of The Atom; Or, The Foundations Of Chemistry.// Easton, PA: The Chemical Publishing Co., 1904. Page 16 [iii] //Atomic Theory// [|www.abyss.uoregon.edu/~js/21st_century_science?lectures?lec05.html] (Accessed Tuesday, October 4, 2011)

[iv] Van Melsen, Andrew G. M. //Atomism.// Encyclopedia Britannica Online. 2011 [|www.britannica.com/EBchecked/topic/41810/atomism/68635/Ancient-Greek-atomism] Visited Wednesday, October 19, 2011. [v] Williams, L. Pearce. //History of Science: the history of science from its beginning in prehistoric times to the 20th century.// [] Visited Thursday, October 27, 2011

[vi] Moran, Bruce T. //Distilling Knowledge: Alchemy, Chemistry, and the Scientific Revolution//. Library of Congress Cataloguing-in-Publication Data, 2005. Pages 2 and 6. [vii] Williams, L. Pearce. //History of Science: the history of science from its beginning in prehistoric times to the 20th century.// [] Visited Thursday, October 28, 2011 [viii] Ibid. Visited Thursday, October 20, 2011 **[ix]** Ibid. Visited Friday, October 28, 2011 [x] Porter, Roy, Park, Katherine, and Daston, Lorraine. //The Cambridge History of Science: Early modern science//. Cambridge University Press, Cambridge, England, 2006. Page 84. [xi] Ibid. Page 83. [xii] Clericuzio, Antonio. // Elements, principles and corpuscles: a study of atomism and chemistry in the seventeenth century //. Kluwer Academic Publishers, Dordrecht, the Netherlands, 2000. Page 12. [xiii] Ibid. Pages 12-13 XIV[] Visited Thursday, November 17, 2011

XV [] Visited Thursday, November 17, 2011

XVI [] Visited Thursday, November 17, 2011 []

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__**John Dalton ~1805**__ John Dalton began his life as the son of an English weaver. He was brilliant from the beginning however, and started teaching at the age of twelve and continued for the rest of his life. He was raised a Quaker, which kept him humble; however his humility never stopped his inquisitive mind. He was a self-taught chemist and studied principally gases. His study of gases led him to come up with postulates concerning Atoms. He was the first scientist to take the idea of an atom from the realm of abstract and make it something quantifiable and feasible. He developed these postulates in the years between 1803 and 1807, and considering what little equipment he had, it is utterly remarkable his postulates hold true to this day with the exception of his postulate stating that atoms cannot be destroyed in a reaction. This has to be qualified later by stating atoms cannot be destroyed in chemical reactions as nuclear chemistry had not yet been discovered. Dalton’s postulates were: With these postulates, Dalton explained several of the simple laws of chemical combination known in his time. These were the law of constant composition and the law of conservation of mass. Dalton also used his postulates to deduce the law of multiple proportions. It seems important to mention that with the extremely limited or non-existent equipment capable of studying atoms at the time Dalton came up with these postulates that much of his ideas were born from deductive reasoning. It bears worth repeating how utterly remarkable it is that not only did he not have the equipment and used deductive reasoning mixed with a strong knowledge of chemistry at the time, he was able to correctly postulate these ideas. Atomic Theory changed significantly in this time. Now the atom had changed from an abstract idea born from an Ancient Greek philosopher to a quantifiable substance. It would take much more time to come up with the numbers to truly define an atom, something that is still ongoing at the present time. Because of Dalton’s discoveries, he is considered the father of modern chemistry and atomic theory. Dalton’s idea of what an atom looked like remained relatively constant until JJ Thomson changed it in 1897. Dalton theorized that an atom was a sphere with no mention or thought to any kind of subatomic particles. All of this began changing when work with different elements and newer equipment came up with experiments that yielded very surprising results.
 * Each element is composed of extremely small particles called atoms.
 * All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.
 * Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.
 * Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

__**William C Roentgen- November 8, 1895**__ One such experiment that was performed with equipment not available in Dalton’s time was the experiment Roentgen performed with a Crookes’ Tube. Sir William Crookes, among others, developed a tube to study the properties of cathode rays, and Roentgen continued the work with cathode rays and Crookes’ Tubes to lead him to discover X-rays. In 1895 he was working in a dark room and sealed a discharge tube to exclude all light, that a paper plate covered with barium platinocyanide became fluorescent when it was placed in the path of the rays from the discharge tube from up to two meters away. Continuing with this experiment, he found that objects of different thicknesses would show up differently when placed in front of a photographic plate. The very first x-ray ever taken was of Roentgen’s wife’s hand, which showed the bones of her hand and the ring on her hand while the skin appeared to be no more than a shadow. Initially this was called a “rontgenogram” but was later labeled the x-ray that we use today. The name x-ray came about because Roentgen didn’t know the nature of the rays, only what would happen when you put your wife’s hand in front of them and a photographic plate, so he called them x-rays. They would later be shown to share the same electromagnetic nature as light, but only differ due to their higher frequency of vibration.

__**Henri Becquerel -1896**__ Becquerel was a contemporary of Roentgen, in fact so are the next few entries on this timeline. He was born into a family of accomplished scholars and scientists and have ever advantage that scientists like Marie Curie and John Dalton did not. Becquerel’s father was a physicist in France whose main focus was studying solar radiation. Becquerel studied a similar but slightly different idea to that of Roentgen. Becquerel discovered natural radioactivity in uranium salts. He was initially studying whether there was any connection to naturally occurring phosphorescence and x-rays. Upon exposure to light, the uranium salts that Becquerel had inherited from his father would phosphoresce, and would fog a photographic plate covered with opaque paper when placed close to it. As luck would have it for the rest of humanity, Becquerel tried this same experiment on a cloudy day and found that the photographic plate was still fogged. It led him to the conclusion that there was a naturally occurring radiation emanating from the uranium salts. Later Becquerel discovered that these radioactive waves were different from x-rays in that they could be affected by both electric and magnetic fields. This must have meant to him that the radioactive particles had a charge to them. Where Becquerel left off in his study of radioactivity, Marie and Pierre Curie picked up.

__**JJ Thomson – 1897**__ Thomson was born and raised in a suburb of Manchester, England. He attended Owens College then began his graduate work at Trinity College, Cambridge where after graduation, he stayed on as faculty. Thomson introduced the next big development in atomic structure. In an original experiment, Thomson used a magnet to bend the light that was in a cathode ray. Typically, light does not bend when a magnet is nearby so Thomson concluded that the light emitted in the cathode ray tube was actually small, negatively charged particles which he called electrons. This was the first addition to Dalton’s atom since Dalton had come up with a model and introduced the first subatomic particle. Seven years later, Thomson would again make a change to Dalton’s concept of an atom.

__**Marie and Pierre Curie – 1898**__ Marie Curie’s story is an inspirational one. After having achieved the highest level of education she could in Poland, she and her sister developed a strategy so that both may earn their degree at university in France. It wasn’t until the age of twenty-four that Curie began her studies at university after having taken years off from studying. She first earned her physics degree, then mathematics degree. All the while, she was living in an attic in the Latin Quarter of Paris, piling on every bit of clothing she had so that she may remain warm enough to sleep at night. Pierre Curie was an internationally known physicist that studied crystals and magnetic bodies at different temperatures. The intellectual curiosity of both Marie and Pierre drew them closer and eventually marriage ensued. A very important contribution that Pierre made to their work was an instrument that could measure very small amounts of electricity, an electrometer. It was constructed by Pierre and his brother and was based on the piezoelectric effect, something that Pierre had actually discovered. Pierre and Marie picked up where Becquerel left off and began to systematically study uranium. They found that thorium also gave off the same amount of radioactivity, and that the strength of the radioactivity differed only with the amount of uranium or thorium present. Marie Curie found that pitchblende contained a substance that gave off radioactivity three hundred times stronger than uranium or thorium. Despite her difficulties in gathering the small amounts of this unknown substance from the large quantities of pitchblende it was found in, Curie eventually gathered a pure enough sample to determine the atomic weight of the substance and was able to declare that radium was a new element. Marie Curie presented this work in her doctoral thesis and won a Nobel Prize for carrying on Becquerel’s work on radioactivity. While studying these radioactive elements the Curies were totally unaware that the radiation being absorbed into their bodies was detrimental to their health. Pierre began to shake uncontrollably and met his untimely death when he was accidentally run over by a horse and carriage. Marie carried on and earned another Nobel Prize in chemistry for the discovery of radium and polonium and continuing her own work in radioactivity. Her continuing work in radioactivity continued to subject her to radiation and Marie died of Leukemia in 1934, very likely induced by the effects of the radiation on her body.

__**JJ Thomson – 1904**__ Thomson again makes the timeline for his advancement of the physical appearance of the atom. Dalton theorized that the atom was a spherical object, with no mention of any kind of subatomic particles. Thomson furthered that after his discovery of the electron by saying that the atom was like plum pudding, the electrons being the bits of plum in the pudding found randomly throughout and the pudding having a positive charge. The middle however was not made up of anything solid, like the pudding and had little physical substance to it. Thomson, throughout his work, was able to establish the charge per mass ration of an electron. This was a number that Robert Millikan was able to use in his experiments to accurately determine the charge of an electron.

__**Robert Millikan – 1908**__ Millikan, an American, was able to determine the charge of an electron by his oil drop experiment. The experiment involved oil droplets falling through a charged plate. Millikan was able to determine the size of the droplets, and therefore the strength of gravity on them as well as their terminal velocity. X-rays were used to ionize the gas in the chamber, making electrons cling to oil droplets and slowing their descent. Millikan discovered that this negative charge was always a multiple of -1.6*E-19, which is the charge on a single electron. Because Thomson had already determined the charge per mass of an electron and Millikan was able to determine the charge of an electron, the mass of an electron was also determined by Millikan.

__**Ernest Rutherford – 1909-1911**__ Rutherford made several advancements in the understanding of the atom, the first of which came about with his gold-foil experiment. Rutherford worked a lot with alpha particles, or positively charged particles that come from a radioactive source. He decided to shoot these alpha particles at an extremely thin piece of gold foil, and based on Thomson idea of what an atom looked like, determined that the positively charged particles would change their course only slightly as they were affected by the attraction of the electrons as the alpha particles shot past them. Some of the alpha particles had an extreme change in their direction, including returning right back to their source. Rutherford determined from this that there must be a tiny but very dense center to the atom which the electrons surround. This led to his planetary model of an atom. Rutherford did not describe how the electrons orbited the nucleus of the atom, as he now called it, but rather said they just surround it. With a small but dense center where the majority of the mass of the atom is found, it was not hard to make a connection to some kind of planetary model.

__**Niels Bohr – 1913**__ Niels Bohr’s contribution to this timeline comes in the form of picking up Rutherford’s slack. Rutherford did little in his model of the atom to explain how the electrons in the atom were behaving. Bohr theorized that electrons were found in circular orbits around the nucleus, and that these orbits came in different shell energies. They were not found in a single layer but were rather found in multiple layers with each layer containing a certain amount of electrons until another shell would be filled. His model of an atom is still used today, despite it being found to be technically incorrect, it still depicts the atom in a fundamental way that is understood by many and allows for it to be easily taught. It should be noted that Bohr, in his career, worked with Rutherford and Rutherford had been a student of Thomson.

__**Ernest Rutherford – 1919**__ Rutherford made another extremely important discovery concerning the atom. In 1919, he performed an experiment by shooting alpha particles (again) at nitrogen gas. His equipment detected hydrogen when he did this. His conclusions were that he was changing nitrogen gas into an oxygen isotope, that the hydrogen nucleus was a basic building block of the atoms and that the hydrogen nucleus was made up of a single, positively charged particle. Rutherford named this particle the proton. He was also the first to artificially change one element into another. It would take thirteen more years until the final piece of the nucleus would be discovered.

__**Erwin Schrodinger – 1926**__ Schrodinger introduced a new model of the atom because he was not happy with Bohr’s. He developed what is known as the cloud model. It basically states that one cannot predict with any certainty where an electron is at any given moment in time, but using probability you can predict where it should be. No longer was an electron relegated to be on a single orbit like Bohr’s model, it was now free to be anywhere in a cloud at any given time.

__**Werner Heisenberg – 1927**__ Heisenberg is best known for two things. At the age of twenty-four he created quantum mechanics. In 1927 he published his well-known Principle of Uncertainty. In it he tried to explain the probability of finding both the velocity and location of a moving particle at any given instant. He said that with the higher precision you could calculate either the velocity or location, the precision of the other calculation decreased. It further explained both the Bohr-Rutherford model of the atom, as well as Schrodinger’s model.

__**James Chadwick – 1932**__ Chadwick solved the last piece of the nucleus puzzle. He discovered evidence of the neutron in the atom when an experiment was performed by two Germans, Bothe and Becker. They used alpha particles to shoot beryllium and they found that an electrically neutral, penetrating radiation was found. Marie Curie’s daughter Irene and Frederic Joliot-Curie both investigated this radiation and found that the only other explanation, aside from neutrons, was gamma radiation. They found this unlikely as the gamma rays would have to be incredibly powerful to displace the protons that were being shot out of the wax that was being shot at. Rutherford also studied the experiment and firmly believed that it must be neutrons. Chadwick worked very hard to prove that it was in fact neutrons, changing the substance being shot and putting it in a chamber to limit ionization. He found that a particle with high neutral energy that had roughly the same mass as a proton was being emitted. Because of his discovery, the gateway was opened for nuclear fission and atomic power. He was awarded the Nobel Prize in 1933.

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