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Dalton's Atomic Theory | #aumsum #kids #science #education #children

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The History of Atomic Chemistry: Crash Course Chemistry #37

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The Creation of Chemistry - The Fundamental Laws: Crash Course Chemistry #3

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Who Was Robert Millikan?

When you think of famous physicists, Robert Millikan may not be the first one that comes to mind. However, his accomplishments - in particular in the area of electric charge and atomic theory - are numerous. Born and raised in the American Midwest by his minister father, Robert Andrews Millikan attended Oberlin College in Ohio as an undergraduate and upon graduation he enrolled at Columbia University. He holds the distinction of being Columbia's first physics Ph.D. recipient.

He spent the majority of his professional career as professor at the University of Chicago and laboratory director at CalTech, teaching and conducting research in the areas of electric charges, the photoelectric effect, and extraterrestrial radiation. He briefly served in the United States Army Signal Corps during World War I, working on meteorological devices. Millikan was married with three sons, all of whom pursued careers in sciences. His religious upbringing led him to give many lectures and write papers over the years detailing his belief that religion and science should be in harmony with one another, as opposed to the opposition and divisiveness the two subjects tend to lend themselves to. Robert Millikan passed away from a heart attack at the age of 85.

The Oil Drop Experiment

The oil drop experiment was perhaps the most famous scientific work of Robert Millikan's career. While at the University of Chicago, he worked with one of his graduate students, Harvey Fletcher, to attempt to measure the charge of an electron. Up until this point in time, around 1913, the world's view of what's going on inside of an atom was primitive by today's standards. Earlier experiments by J.J. Thomson had shown that atoms contain some type of negatively charged particle, and that these particles were of a mass significantly smaller than that of a hydrogen atom. Exactly what the magnitude of that negative charge was remained unknown. It also wasn't completely accepted yet that subatomic particles (particles smaller than an atom, that are contained within an atom) even existed, so the whole field of atomic theory was really coming of age in the early part of the 20th century. It was the goal of Millikan and Fletcher to quantify the charge of an electron, and they more or less succeeded, as you will see.

The experiment itself was somewhat simple in design, when you compare it to modern day particle accelerators and such. It is called the oil drop experiment because it involved just that - dropping oil between electrodes (electrically charged plates). The force of those droplets passing through the electric field was measured, as was electrical field between the electrodes, and in measuring and calculating those quantities, they were then able to determine the charge on a single electron. After numerous trials, and standardizing their measurements since not all oil droplets were exactly the same size, Millikan and Fletcher concluded that the charge of an electron, or the elementary charge (e), was equal to 1.5924(17) x 10^-19 C. Coulombs (C) are the SI unit of electric charge, e is used to designate electron charge in lieu of writing out the entire numerical value each time it is used, and the final two parenthetical digits represent the accepted uncertainty in the final decimal places. Not only were the pair able to conclusively quantify an electron's charge, e, but in doing so were able to prove that electrical charges are quantized(single, discrete values), as opposed to continuously changing values as was the conventional thought at the time. It was truly a groundbreaking experiment.

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atomic theory & oil drop experiment - video & lesson transcript | study.com

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Robert Millikan: Biography, Atomic Theory & Oil Drop Experiment - Video & Lesson Transcript | Study.com

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When you think of famous physicists, Robert Millikan may not be the first one that comes to mind. However, his accomplishments - in particular in the area of electric charge and atomic theory - are numerous.

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In approximately 1909, Robert Millikan conducted his well known "Oil Drop Experiment" with colleague, Harvey Fletcher. Together, the two began trying to measure the charge of an electron by placing charges on drops of oil and measured the strength needed of an electric field to keep the drop from falling.  Millikan knew the weight of the drops he used and could figure out how much gravitational force was acting on the drop, so ultimately he could establish what electric charge the drop had. No matter the charge on the different drops,  Millikan concluded that it would always be a multiple of -1.6 x 10 -10 C.
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In 1793, Joseph Proust came up with the Law of Definite Proportions, which is the idea that the element ratio in a compound is always the same. This law was made as a result of his concerns with inorganic binary compounds, which are compounds that lack carbon and have exactly two different elements, like  sulfates, sulfides, and metallic oxides. His research was then published in 1795 in Spanish journals. This proved to be helpful for Dalton, for he later proposed the Law of Multiple Proportion and eventually his atomic theory, which was aided by Proust's research on the matter. However, Proust was not supported by everyone, for French chemist, Claude-Louis Berthollet, argued against many of Proust's points. The controversy was ended with Dalton's previously mentioned atomic theory.
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In 1774, Antoine Lavoisier began to experiment with fire and studied what happened to an object after it had been burned along with the rest of the process of burning. This was a very significant study because from it derived The Law of Conservation of Mass, which states that matter can neither be created or destroyed in either a physical or chemical reaction, which birthed the beginning of modern chemistry.
Fun Fact: Antoine Lavoisier was beheaded in 1794 for his relationship with the daughter of an executive of a tax firm.
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The "quantum" or "electron cloud" theory has been conceived from many years of research and studying and took many scientists to develop, who have come to the conclusion that an atomic model is based off of wave quantum mechanics. These principles proved Bohr wrong in that electrons do not follow a definite path and are actually impossible to know exactly where they are at any given time. However, scientists can produce a rough estimate of where an electron may be based on the amount of energy it possesses. As thought by previous scientists, an atom consists of a nucleus surrounded by enough electrons to make the atom neutral. Although the electrons do not move in a definite pattern as they swirl around the nucleus at a very fast pace, they do not travel in random movements; electrons with high energy can be found on the outskirts of an atom, but electrons with low energy can be found closer the nucleus.
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After observing and studying the different theories of atomic structure, Niels Bohr decided to take it a step further by putting each of the electrons into their specific energy level in 1913. According to Bohr, electrons followed definite orbits around the nucleus of an atom, these orbits were dependent upon the amount of energy present in a particular electron.
Bohr also presented that when electrons quickly move from place to place,  they emit or absorb radiation.
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John Dalton was the first scientist with evidence to prove the atomic theory. He is known as the father of atomic science. He believed that everything is made up of atoms, which vary between elements. He thought atoms had no smaller parts, and could not be split or created. He also believed that by the arrangement of atoms, different elements could be created.
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Aristotle was a rival philosopher of Democritus, who, with his student Plato theorized that everything in the universe was made up of four elements: fire, water, earth, and air. Although he was completely incorrect, he was more popular and his theory was accepted throughout ancient Greece.
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In 1908, Earnest Rutherford began his work to find answers as to what an atom is and what its components are. To do so, he shot a line of positively charged particles at gold foil that was only 2,000 atoms thick, hence the name "Gold Foil Experiment." In this experiment, most of these particles passed through the sheet of gold. However, occasionally the positively charged particles would bounce back towards the gun. This suggested to Rutherford that there are positively charged ions in the gold sheet because positive particles bounce off of each other as if they were solids. Contrary to previous beliefs, atoms did not resemble plum pudding, but are instead mostly open space. Rutherford established that there is a nucleus in atoms, which is very small, dense and contain the positively charged particles, whereas the electrons were dispersed around the edge.
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Beginning in 1897, J. J. Thompson provided the idea that atoms are made of even smaller particles than what was previously thought. He found out this information by observing how how electric currents travel through different gases, and he found that as they travel through these gases, they gave off rays that consisted off negatively charged particles. From this, Thompson concluded that  the negative charges he was seeing were coming from within the atom, because the gas was uncharged, so atoms were more divisible than they had thought. From this, he was able to make a comparison for the average person to comprehend what an atom was like and compared that structure of an atom to that of plum pudding.
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Democritus was a Greek philosopher around 400 B.C. who thought that if one were to cut objects, there would be a point in which it could not be cut any more. He called these parts Atomos, and theorized that they would be spherical in shape and indivisible. The shape and size would determine what type of substance it created.
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