(TG) October 13, 2010: Atomic Theory

THE ATOMIC THEORY

ARISTOTLE:

• One of the first theories ever created was called the "Four Elements Theory"
• Consisted of: Water, Earth, Wind, and Fire
• Lasted about 2000 years, but is not currently a valid theory because it cannot be proven 

DEMOCRITUS: 

• In 300 BC Democritus said atoms were indivisible atoms
• It was the first mention of atoms: atomos
• Was only a conceptual model 
• No mention of a nucleus
• Does not explain Nuclear Reactions

LAVOISIER (LATE 1700'S)

• Created the law of the conservation of mass: "in a chemical reaction no matter is destroyed or created"
• Also had a law of definite proportions: (e.g. in a H20 molecule there is always 11% H and 89% O)

PROUST

• In large proportions atoms have the same ratios as they do in molecules or compounds
• Proved Lavoisiers Laws correct 

DALTON

• Believed atoms are solid, indestructible spheres (e.g. billiard balls)
• This theory provides an explanation for different elements
• Based on the law of conservation of mass (if atoms are not destroyed, mass doesn't change)
• Having a molecule explains the law of of Constant Composition
• 2H2     + O2             =      2H2O 

J.J. THOMPSON

• Raisin Bun model
• solid, positive spheres, with negative particles embedded in them
• first atomic theory to have positives and negatives
• demonstrated the existence of electrons (cathode tube)  

 RUTHERFORD's assistant (1905) (

• showed that atoms have a positive, dense, center, with electrons outside of it
• resulted in planetary model
• explains why electrons spin around nucleus
• suggests that electrons are made up of mostly open spaces

RAISIN BUN MODEL

RUTHERFORDS MODEL (ELECTRON SPACING)

COMPARING THE MODELS