October 4th - An Introduction to Atomic Theory

This class marks the beginning of the second unit, where we learn all about atomic theory!

The class started with a brief discussions of the graphing homework. Shortly after, the class broke into groups and we wrote down any preexisting knowledge we had about atomic theory. After everybody had a chance to jot some of their own notes, we discussed the topic as a class.

We learned that the very first atomic 'theory' was proposed by Aristotle around 300 B.C.E. He thought that all matter consisted solely of Earth, Air, Fire, or Water (the four elements). However, it is important to note that his theory was not testable, and its classification as an actual scientific theory has been questioned. 

Around the time of Aristotle,  a Greek philosopher, Democritus, proposed the idea that all matter was made of particles. His 'theory' also stated that all matter had a finitely smallest particle. He called this particle atomos, from which we get the word 'atom'. Again, it is important to note that this theory was not testable either. Other philosophers also questioned his theory, asking how the particles stuck together. Democritus could not give an answer.

Antoine Lavoisier, known as the father of modern chemistry, and Joseph Louis Proust (both alive in the 1700s) significantlty updated the atomic theory. Each had an important contribution. Lavoisier proposed the law of conservation of mass, which states that no matter can be created of destroyed. He theorized that matter can never be 'used up' in a chemical reaction; it can only be rearranged. Proust proposed the law of constant composition/definite proportions, which states that the same elements are always found in the same proportions in compounds. For example, wherever water is found in the universe, it will always exist as 88.9% oxygen and 11.1% hydrogen. These discoveries were significant, and still play a large role in our understanding of matter. However, it is (again) important to note that Lavoisier's theory was not exactly true. In fact, energy, not mass, is always conserved in a reaction. However, mass is simply a concentrated form of energy, so the principle remains true. Below, you can find a video of explaining the law of conservation of mass:

Around the turn of the 19th century, John Dalton proposed his atomic theory of matter. This theory stated that all matter is composed of small particles (called atoms) and that each atom of an element is identical, yet different from those of other elements. He restated the laws of conservation of mass and constant composition in his theory. However, his proposal still had flaws. He thought that atoms were hard, round balls and were the smallest parts of matter. He also couldn't explain the reason why the elements acted differently from one another. 

About one hundred years later, J.J. Thomson presented his raisin bun model of the atom. He believed that atoms were composed of both positive and negative parts. Using a cathode ray tube, he discovered and named the negative particles in atoms, calling them electrons. He theorized that they were embedded in the positive part of the atom, like how raisins are embedded in the bread of a raisin bun. However, we now know that electrons aren't actuallly embedded in atoms. Instead, they exist in shells. We also know that atoms are mostly empty space, not full of matter. Below is a picture of Thomson's model:

 

The last model we learned about was Rutherford's planetary model. With the results of the famous gold foil experiment, he proposed that each atom had a nucleus; a positive, solid, dense core of what we now know to be protons and neutrons. He proposed that electrons existed outside the nucleus.  His model also suggested that the atom was mostly empty space. In the video below, you can see the experiment that led to the discovery of the nucleus:

That was all the material we covered for the day. However, we aren't done learning about atomic theory yet. We still have to learn about the findings of Niels Bohr! Unfortunately, that has to wait until next class...

Posted by Michael.