1. DALTON’S ATOMIC THEORY
UNIT #2 THE ATOM

2. Background
At the time of John Dalton, chemists had this basic understanding:
Elements react with other elements to form compounds
Elements cannot be broken down into other substances
They had no idea that atoms (the individual particles that make up matter) and elements and compounds were related
Dalton drew this connection between atoms and elements and compounds

3. Inspiration from Proust’s work
Dalton, after reviewing Proust’s research, hypothesized that elements were made up of different types of atoms  the type of atom determined the element
Compounds were combinations of these elements which had reacted

4. Evidence for Dalton’s conjecture
After Dalton discovered the Law of Multiple Proportions, he was convinced his hypothesis about atoms were true
The Law of Multiple Proportions is the following: When two elements react to form different compounds, the ratios of the second element combined with one gram of the first element can be reduced to whole number ratios

5. Illustrating the Law of Multiple Proportions
Mass of Nitrogen
Mass of Hydrogen
Compound A
1 g
0.144 g
Compound B
0.216 g
Compound C
0.024 g

6. How does this data relate to atoms?

7. Dalton’s Atomic Theory
Consists of four postulates
All matter is composed of small, indivisible particles called atoms
All atoms of the same element are the same, and all the atoms of different elements are different
Atoms break apart and rearrange themselves to form new substances
Atoms combine in simple, whole number ratios to form compounds

8. Dalton’s Flaws