Atom Theory Development Timeline

By: Luke Hargadon

460 BC- Democritus• Democritus was a Greek philosopher

who, under the influence of hallucinogenic drugs, attempted to crush these drugs for days on end. He eventually reached a point where he could no longer make the pieces any smaller. He theorized that everything broke down into a substance called “atomos” which roughly translates to indivisible. This idea that all mater is made of small, unbreakable pieces is the foundation for all atomic theory.

1803- Dalton• Dalton is considered the

father of the modern atomic theory. He took the philosophical notion of the atom and intended to prove it using scientific evidence. He created o model of the atom known as the “Billiard Ball” model. This model theorizes that atoms are in fact tiny, unbreakable spheres.

1869- Mendeleev• Dmitri Mendeleev contributed a lot to

the theory of atoms by being one of the first people to organize all known elements of the time into a table based on a few qualitative and quantitative observations of these elements. He found a better way to find the atomic masses of the different elements and used this knowledge to organize a table of the known elements of the time. He organized them using their atomic masses in conjunction with the elements’ properties in two columns, leaving many spaces in between groups of elements as a consequence.

1885- Eugene Goldstein• Eugene Goldstein's influence on the

atomic theory comes from his discovery of one of the subatomic particles which serve to identify all elements and make up all mater in the universe. He used cathode ray tubes and discovered that there were two beams of particles going in opposing directions. He theorized from this that the tow opposing beams of particles must have opposite charges. From this he discovered the existence of the particle known as the proton. This particle would end up being a crucial cornerstone in the study of the atom and the atoms of different elements, most importantly when it comes to classification.

1896- Henri Becquerel• Henri Becquerel contributed to

the atomic theory by observing certain elements and finding something peculiar about them. He observed that certain elements give off an unseen energy which he called at the time Becquerel energy. He discovered radiation and the three different classifications of nuclear radiation. He found that different types of elements release energy and decay into other elements.

1897- JJ. Thompson• JJ. Thompson is widely recognized for

having a large influence on the ideas of the atomic theory. He found that all neutral partials of the same atom have the same number of negative particles. He also created a new model for the structure of the atom. This structure of atom is known as the “plum pudding” model. This model describes an atom as a glob of protons and neutrons in which the electrons are suspended. This was the model that would for a while supersede the billiard ball model until refined and replaced.

1907- Ernest Rutherford• Rutherford was one of the major

influences in the modern atomic theory in that he discovered one of its key elements. In his experiments, Rutherford took a piece of gold and hammered it out until it was transparent and then shined a laser on it. The laser appeared to bounce off the surface of the foil. From this observation Rutherford deduced that there must be a very small and dense particle in the center of the atom which is able to deflect the light. He called this particle the nucleus and it would serve as one of primary elements of future atomic models.

1910- Millikan• Millikan was one of the

pioneers of atomic theory. He was able to use preexisting mathematics in order to fang the mass of the electron. This information would prove valuable as more research on the nature of subatomic particles are conducted.

1913- Neils Bohr• Neils Bohr made huge

contributions to the atomic theory. He created a model of the atom which is the most well known and most accurate in history. This model involves a core which is the nucleus with orbits around the core of the electrons. This model would be used to make studying the atom easier and give a better understanding of its operation.

1913- Frederick Soddy• Frederick Soddy had a substantial

influence on the modern atomic theory. Frederick theorized and proved that two atoms of the same element could have different masses. He called these different forms of elements isotopes and found that they had everything in common with their regular counterparts except for an increased number of neutrons. His worked proved that the mass properties of atoms are not absolute and this would change the way in which elements needed to be studied from then on.

1925- Heisenberg• Heisenberg made a large contribution to

the field of atomic theory with his ideas about the nature of subatomic particles in motion. He created a principal known as the “uncertainty principal”. This principal described the idea that as one gets closer to level of atoms and below the rules of motion start to warp. We cannot exactly predict the movement of objects on the sub atomic level. It can only be calculated using a probability function to determine where it is most likely to be. This discovery has influenced the way that studies of objects at the sub atomic level are conducted. http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec14.html

1932- Chadwick• Chadwick made huge strides

towards the human knowledge of the atom through his discovery of the third sub atomic particle. Chadwick took the atomic masses of the element and noticed that the number of proton was not equal to it and that there was something missing. From this he found the existence of neutrons which would make are understanding of one layer of subatomic particles whole.

Picture Citations (In Order of Appearance)

• http://www.quotecollection.com/image-view.php?img=democritus-3.jpg• http://fcit.usf.edu/florida/teacher/science/mod3-resources/lecture2.htm• http://www.nndb.com/people/593/000091320/• http://www.xtimeline.com/evt/view.aspx?id=629217• http://www.nndb.com/people/608/000072392/• http://www.nndb.com/people/479/000099182/• http://

www.nobelprize.org/nobel_prizes/chemistry/laureates/1908/rutherford-bio.html

• http://www.aip.org/history/gap/Millikan/Millikan.html• http://csep10.phys.utk.edu/astr162/lect/light/bohr.html• http://www.nndb.com/people/286/000099986/• http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec14.htmlhttp://www.aps.org/publications/apsnews/200705/physicshistory.cfm