Atomic Theory

In this next chapter, we will cover Atomic Theory and the Periodic Table and trends.

First, let's look at some history behind the Atomic Theory.

In the ancient times, Greek philosophers suggested that matter was made of atomos, which means the smallest pieces of matter.

At around 400 BC, Democritus was the first to propose this, and that atoms were indivisible particles.



Aristotle, a famous philosopher at the time, disagreed with this theory and thought that matter was made of earth, air, fire and water. This theory became true for 2000 years, but since it was only conceptual (much like Democritus's), scientists were able to disprove the theory later on.



In the 1700s, a French chemist named Lavoisier stated the Law of Conservation of Mass and the Law of Definite Proportions. These laws suggested that in a compound of say, H2O, there will always be 11% Hydrogen and 89% Oxygen.



In 1799, Joseph Proust experimentally proved Lavoisier's laws, and added that when a compound is broken down, products will exist in the same ratio as in the compound.



Then, in the early 1800s, John Dalton developed the basis of the modern Atomic Theory. He suggested that:

1. Elements were made of tiny indestructible spheres called atoms.
2. All atoms of an element were the same.
3. Atoms of a given element can be differentiated from another element by its relative atomic weights.
4. Atoms of one element will combine with atoms of other elements to create compounds.



In the 1850s, J. J. Thomson created an experiment known as the Raisin bun model (or the Plum Pudding). This model had solid, positively charged spheres, as well as negatively charged spheres. Using this model, he proposed that a unit over 1000 times smaller than the atom, known as the electron, existed.



A student of J. J. Thomson, Ernest Rutherford discovered that atoms have a positively charged, dense center with electrons surrounding it on the outside. He explained why electrons spun around the nucleus, but he could not explain why the electron did not fall into the nucleus and destroy the atom.





Luckily, a partner of Rutherford's, Niels Bohr found the solution. Bohr was studying gaseous samples of atoms at the time, and came to the conclusion that electrons surrounding the nucleus were in specific energy levels. When the electron was excited, it would jump to a higher level. When an electron came back down, it would release energy in the form of light. Each of these jumps gives off light in different wavelengths; therefore creating different colours, as the colours ROYGBIV all have different wavelengths.





In conclusion:
An electron surrounded the nucleus, and had a negative charge. If its mass was 1, then the mass of protons would be 1836, and the mass of neutrons would be 1837. Protons, positively charged particles, along with neutrons, neutral particles, were in the nucleus of an atom.

This is a really good website to check out:
http://chemistry.learnhub.com/lesson/3663-history-of-the-atomic-theory-i-ancient-times
Now a video to summarize!!