The Atomic Theory (and its looong history!)

When I left Chemistry today, I felt as though the last hour that I'd spent in the room at the corner of the science wing wasn't a Chemistry lesson, but a history lesson! We learned about the atomic theory and its looong history!





It all started out with Ancient Greece's (and other civilizations with similar theories) 4 element theory. This stated that 4 elements exist: Earth, Air, Fire and Water. This, of course, could not be tested, but with the technology then, it was the best guess they could give!




  
 

Next was the theory of Democritus in 300 BC, that stated that atoms are indivisble particles. There was no mention of the nucleus or constituents yet, and it couldn't explain atoms' roles in chemical reactions. This  theory was also not testable. 

Doesn't he look like such a nice fellow?

Next was Lavoisier's theory in the 1700's. He created:

-Law of Conservations of mass: There has to be the same number of atoms of each element in the reactants as there is in the product of the chemical reaction. 

-Law of definite proportions: Compounds have different proportions based on mass, for example water is always 11% hydrogen and 89% oxygen (even though there are 2 hydrogens for each oxygen atom, oxygen has a greater mass)

Now, is he working hard or hardly working? 

 

Next was Proust with his theory in 1799. He stated that if a compound is broken down into its constituents, the products have to exist in the same ratio as it did in the compound. I guess this would be like in decomposition, for example H20 -> H2 + O2  this would have to be balanced as 2H20 -> 2H2 + O2.

 

Next in line is Dalton in the 1800's who stated that atoms are solid, indestructable spheres, and that each atom provides for a different element (different spheres). This was based on the law of conservation of mass.

What a great person.

 

Next is JJ Thomson in the 1850's who created the Raisin Bun model. No, he was not a bakery chef, but he found that atoms are solid, positive spheres that have negative particles embedded in them. This was the first theory that stated atoms have positives and negatives (protons and electrons) in them. This WAS tested using a cathode ray tube! 

Well doesn't he just look like the friendliest person in town.

 

Rutherford was next (and last, on this list!) in 1905, and he showed that atoms have positive dense centres! Well where do the negatives go, he thought? This resulted in a planetary model (centre with 'orbit rings' around it) which explains why electrons spin around the nucleus, and this also explains that atoms are mostly empty space!

I think he's in the middle of a 'eureka' moment here!

Quite fascinating, I know. Shows the long history of chemistry, and how technology can help support our theories!

Here's a video that outlines the people that we learned about, and it's even thrown in a few extra names that aren't on this list! 

Atomic Theory History Lesson