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History of the Atom

Chemistry Timeline Project

Stefanie Williams

on 11 April 2014

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Transcript of History of the Atom

History of the Atom Timeline
Project by Stefanie Williams
Chemistry Honors - Mrs. McCormick
2013-2014 School Year

Current Event Scientific Event
Current Event Scientific Event
Theory of Matter
400 B.C. - Democritus
Democritus was a Greek philosopher
During his time, it was believed that there were four elements and Democritus agreed with this theory.
However, he proposed that there was a limit to how small an element could be divided.
Democritus stated that matter may appear to be smooth and solid from afar, but, when magnified, it could be seen that matter is made up of very small pieces.
He said that elements were also made of very small pieces that eventually couldn't be divided anymore.
These pieces that couldn't be divided anymore were named atoms, meaning indivisible in Greek.
This theory became known as the Discontinuous
Theory of Matter.
Around the same time...
Greek engineers created the first catapult which would lead to many new advances in weaponry and technology over the years.
At the Same Time...
The people of Latium sent a letter to Rome's government requesting to combine with Rome while keeping both empires equal. Rome, being more powerful, refused their request and the Latin War began between Rome and Latium.
That Same Year...
On March 4th, the first U.S. congress met and put the Constitution into effect. This Congress was made of 22 people, 9 senators and 13 representatives.
That Same Year...
That Same Year...
That Same Year...
That Same Year...
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That Same Year...
That Same Year...
That Same Year...
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That Same Year...
That Same Year...
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On January 8th, the eleventh amendment to the U.S. Constitution was ratified. This amendment was the first outside of the Bill of Rights and helped to form and recognize the judicial powers in the United States.
On February 24th, the Supreme Court made the final decision on the Marbury VS Madison case. This is the first time that the Supreme Court ever ruled a law unconstitutional.
On August 9th, horses were in Hawaii for the first time in history.
On April 3rd, the Pony Express began between St. Joseph Missouri and Sacramento California.
On February 12th, the first artificial ice rink opened in North America. This ice rink was located in Madison Square Garden in New York City.
On April 14th, Thomas Edison's Kinetoscope is shown to the public for the first time. The kinetoscope was his invention with moving pictures.
On March 11th, a meteorite entered the Earth's atmosphere and exploded over New Martinsville, West Virginia. There were no injuries reported, but the debris from the explosion caused damage to the area.
On January 3rd, the word "automobile" was used for the first known time in an editorial in the New York Times.
On March 5th, the American Hall of Fame was first founded and opened.
On January 4th, the Supreme Court Ruled that all Puerto Ricans must be admitted into the United States under all circumstances.
On February 15th, the Boy Scouts of America was officially founded and has grown into the huge organization that it is today.
On January 10th, the first photograph was taken from an airplane above San Diego, California.
On February 8th, "Disney Brothers Cartoon Studio" officially became known as "Walt Disney Studios."
On March 2nd, Babe Ruth's salary was increased to a whopping $70,000 per year, making him the highest paid baseball player of the time.
On January 17th, Popeye appeared for the first time in the comic strip "Thimble Theater."
On July 15th, the 31st President of the United States, Herbert Hoover cut his own salary by 15%.
Continuous Theory of Matter
340 B.C. - Aristotle
Aristotle didn't agree with Democritus' Discontinuous Theory since it contradicted his religious beliefs.
In Aristotle's eyes, the inability to divide an atom was a direct restriction on the all-powerful gods. In his opinion, if the gods wanted to divide an element to make it smaller than an atom, then they should be able to do so.
Essentially, Aristotle's theory was simply that there was no such thing as atoms and that no element or piece of matter could ever be too small to be divided.
Although we now know that Democritus Discontinuous Theory of Matter was actually correct, Aristotle's Continuous Theory of Matter was upheld for about 2,000 years.
Law of Conservation of Matter
1789 - Antoine Lavoisier
From much experimentation, Lavoisier formed the Laws of Conservation of Mass, or the Law of Conservation of Matter as it is also known as.
This law states that no matter what change occurs, the total amount of mass will remain the same throughout.
In other words, substances can be changed, combined, or separated, and the overall mass will always be the same amount.
Law of Definite Proportions
1798-1804 - Joseph-Louis Proust
The Law of Definite Proportions, or Proust's Law as it is often referred to as, is a very simple law that is pivotal in chemistry.
This law essentially states that a compound always has exactly the same proportion of elements in its mass.
In other words, if you have multiple samples of the same chemical compound, then each sample will have the same proportions of elements based on mass.
Take water for example. Pure water is made up of hydrogen and oxygen. Oxygen accounts for 8/9 of the mass of water, leaving hydrogen with the remaining 1/9 of the mass. Because of what is stated in the Law of Definite Proportions, the mass of every sample of water will have this exact distribution of mass.
Law of Multiple Proportions
1803 - John Dalton
The Law of Multiple Proportions, otherwise known as Dalton's Law, consisted of several parts.
The first part was that when elements combine, they combine in a ratio of small whole numbers. In other words, one carbon atom will not combine with half an atom of oxygen.
For example, when oxygen and nitrogen react and form a compound, they can form nitrate or nitrite. So, they form NO2 or NO3, but they don't form NO2.52.
The Law of Multiple Proportions also states that, if two elements can form multiple compounds, then the ratios between the masses of the compounds will be written in small whole numbers.

Dalton's Atomic Theory
1803 - John Dalton
Although Dalton's atomic theory was made two hundred years ago, the points made in the theory remain to be true today. There are four main points made in his atomic theory. They are as follows:

1) All matter is made of atoms. Atoms cannot be divided and are indestructible.
2) All atoms of the same element are identical. They all have the same mass and the same exact properties.
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is simply a rearrangement of atoms.
Light Travels as a Wave
1860s - James Maxwell
Crookes' Tube
1879 - William Crookes
Cathode Ray Tube
1894-1897 - J.J. Thomson
1897 - J.J. Thomson
Discovered Electron
Gold Foil Experiment
1899 - Ernest Rutherford
1900 - Max Planck
Light Travels as Particles
Plum Pudding Model
1904 - J.J. Thomson
1910-1913 - Robert-Andrews Millikan
Charge and Mass of an Electron
Planetary Model of an Atom
1911 - Niels Bohr
1926 - Erwin Schrödinger
Quantum Mechanical Model of the Atom
Uncertainty Principle
1927 - Werner Heisenberg
1929 - Louis de Broglie
Wave-Particle Duality
Discovered Neutron
1932 - James Chadwick
Modern Atomic Theory
The development of the atomic theory that we currently have obviously took many years and countless people to build. Scientists are still researching and experimenting with atoms today and we may never really know everything about atoms.
The first theory of how light travels was made by James Maxwell in the 1860s. Maxwell thought that light traveled as a wave.
He performed many experiments relating to electromagnetic theory and the way light travels.
From the observations he made during these experiments, Maxwell formulated a set of equations (known as "Maxwell's Equations") that display the properties of electric and magnetic fields, showing that light is an electromagnetic wave.

Over the years, there were three main theories about how light travels, and, although they were all slightly different, all three were correct in one way or another.
The Crookes' tube was created by William Crookes in 1879 to show the path traveled by cathode rays. A cathode ray is a beam of electrons shot from a cathode in a vacume. The tube was a sealed glass tube, and, when an electric current came into contact with the tube, a bright fluorescent light appeared on the walls of the tube. This light was accredited to the interaction of electrons with the gas in the tube.
While experimenting with cathode rays, J.J. Thomson made a discovery that changed the entire study of the atom. Prior to Thomson's discovery, it was thought that the atom was the smallest piece of matter and that nothing could divide it or be smaller than it. However, Thomson proved that theory to be wrong.
He realized one day that the rays from the cathode ray tubes could actually be streams of particles. However, they were much smaller than atoms and they appeared to be miniature pieces of atoms. Thomson named these particles "corpuscles" and proposed that they could possibly make up atoms.
Thomson and others continued to experiment with this hypothesis, and Thomson's theory turned out to be correct. The rays from cathode rays and atoms are made up of electrons.
Rutherford's gold foil experiment gave scientists a whole new perspective of the atom. In the experiment, radioactive particles were shot through very thin metal foils (specifically gold). These particles were then detected by screens coated with zinc sulfide. You can see this in the image below.
When experimenting, Rutherford found that most particles went straight through the foil and one in eight thousand was deflected. This caused Rutherford to conclude that most of an atom was made up of empty space.
After this experiment, Rutherford published an atomic theory that stated that the atom had a central positive nucleus and it was surrounded by negative orbiting electrons. There was also a lot of empty space in his model.
Today, we know all of Rutherford's inferences that he made to be true.
In 1894, J.J. Thomson began a long series of experiments that would lead to several important discoveries. During his experiments, Thomson revised the cathode ray tube. He put deflector plates inside of the cathode ray to experiment with. With his altered cathode tube, Thomson attempted to deflect cathode rays with an electric field and was successful. He was then able to infer multiple things and make many discoveries in the future because of his experimentation with cathode ray tubes.

Max Planck is thought to be the founder of quantum theory. In 1900, he realized that light and other electromagnetic waves actually were let off in packets, or particles, of energy. He named these packets "quanta." These quanta could only come in certain values that were multiples of a constant, h, that is now referred to as the "Planck Constant." This is usually thought of a step towards quantum theory, which has changed the whole perspective of science and of the world.

The Plum Pudding Model of the atom was proposed before the discovery of the nucleus. In this model, the atom is made of negatively charged "plums" surrounded by positively charged "pudding." The electrons moved freely and rotated within the positive mass.
By Millikan's time, the charge of electrons was already known. By bending electrons in electric and magnetic fields, other scientists determined that they were negative. It was also known that the ration of the charge to the mass was e/m.
However, the mass of electrons was still not known. In his experiments, Millikan attached electrons to water droplets and used commonly known formulas to determine the mass. He specifically found the velocity of the water droplets as they fell and used the information in that to determine the mass. He also showed the charge to be a whole number multiple of 1.592 x 10⁻1⁹C. Today, the accepted value is 1.602 x 10⁻1⁹C.

The Planetary, or Bohr, model of the atom is one of the most well recognized. the neutrons and protons make up the nucleus in the center and the electrons are on orbitals that orbit the nucleus as the planets orbit the sun.
Erwin Schrodinger created the quantum mechanical model of the atom. In this model, electrons are defined as standing waves.
The Uncertainty Principle states that the position and speed of a particle cannot both be known. Either you know the speed or you know the position of the particle. Werner Heisenberg stated this in 1927.
De Broglie first stated the idea of a wave-particle duality in his doctoral thesis. His thesis was then passes onto Einstein who agreed completely with it and endorsed the concept. De Broglie was able to figure out the duality by combining equations and ideas to get a concept that worked.
The neutron was not discovered until 1932. In 1932, Chadwick used scattering data to find the mass of the nucleus. It had been thought that the nucleus was made of protons and electrons, but the electrons couldn't be bound. Chadwick saw this and, after some experimentation, proved that electrons weren't in the nucleus because neutrons were.
Modern Atomic Theory consists of everything that we currently know about atoms. Basically, every discovery made contributed to atomic theory, and it's so vast of a topic that it's almost impossible to know where to begin. Subatomic particles...elements... various orbital types... they're all a part of the modern atomic theory. And, as time goes on and new discoveries are made, the modern atomic theory will continue to expand.
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