1. CHEMISTRY TIMELINE #1
1800's
Joseph Proust: The law of definite proportion (composition)
John Dalton: The Atomic Theory, The law of multiple proportions
Joseph Gay-Lussac: Combining volumes of gases, existence of diatomic molecules
Amadeo Avogadro: Molar volumes of gases
Jons Jakob Berzelius: Relative atomic masses, modern symbols for the elements
Dmitri Mendeleyev: The periodic table
J.J. Thomson: discovery of the electron
Henri Becquerel: Discovery of radioactivity
1900's
Robert Millikan: Charge and mass of the electron
Ernest Rutherford: Existence of the nucleus, and its relative size
Meitner & Fermi: Sustained nuclear fission
Ernest Lawrence: The cyclotron and trans-uranium elements

2. Development of Dalton’s Theory
(Lavoisier, 1794)
The Law of Conservation of Mass:
“The total mass of materials before and after a chemical
reaction must be the same.
For example, if we combine 89 grams of oxygen with
11 grams of hydrogen under the appropriate conditions,
100 grams of water will be produced—no more and no less.”

3. Lavoisier heated a measured amount of mercury to form the red oxide of mercury. He measured the amount of oxygen removed from the jar and the amount of red oxide formed. When the reaction was reversed, he found the original amounts of mercury and oxygen.

4. Development of Dalton’s Theory
(Proust, 1797)
The Law of Constant Composition:
“Any given compound always consists of the same
atoms and the same ratio of atoms.
For example, water always consists of oxygen and
hydrogen atoms, and it is always 89 percent oxygen
by mass and 11 percent hydrogen by mass”

5. Law of Definite Composition:
A compound always contains two or more elements combined in a definite proportion by mass.
Assign carbon a mass of 12.
Carbon reacts with oxygen to form CO giving the following proportion by mass:
O/C = 1.33
If C is 12 then O must be 12 X 1.33 or 16.

6. Law of Definite Composition: (Cont. )
Magnesium always combined with oxygen in the proportion:
Mg/O = 1.5
If O is 16 then Mg must be 16 X 1.5 or 24.
Example:
What is the relative mass of sulfur if it combines with magnesium in the following ratio?
Mg/S =
Chemists used proportion problems like those above until all of the elements were assigned a relative mass.

7. Development of Dalton’s Theory
The Law of Multiple Proportions:
“If two elements combine to form more than one compound, the masses of one of the elements that can combine with a given mass of the other element are related by factors of small whole numbers”
For example, water has an oxygen-to-hydrogen mass ratio of 7.9:1.
Hydrogen peroxide, another compound consisting of oxygen and
hydrogen, has an oxygen-to-hydrogen mass ratio of 15.8:1.
The ratio of these two ratios gives a small whole number.

8. Law of Multiple Proportions:
Atoms of two or more elements may combine in different ratios to produce more than one compound.   
When carbon combines with oxygen it does so to produce two different ratios by mass.
Compound 1 = 1.33g of O for every 1g of C
Compound 2 = 2.66g of O for every 1 g of C
2.66/1.33 is a ratio of 2:1
This can be explained if compound 1 is CO while compound 2 is CO2

9. Law of Multiple Proportions: (Cont.)
 When oxygen combines with nitrogen it does so to produce three different ratios by mass. 
Compound A = g of N for every g of O.
Compound B = g of N for every g of O.
Compound C = g of N for every g of O.
Notice that….
Compound A contains twice as much nitrogen per gram than compound B
Compound B contains twice as much nitrogen per gram than compound C
This leaves us with several possibilities…..
compound A N2O or NO or N4O2
compound B NO NO N2O2
compound C NO N2O N2O4

10. Dalton’s Atomic Theory
All matter is composed of extremely small particles called atoms
Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties
John Dalton
Atoms cannot be subdivided, created, or destroyed
Atoms of different elements combine in simple whole-number ratios to form chemical compounds
In chemical reactions, atoms are combined, separated, or rearranged

11. Here are some of the symbols Dalton used for atoms of elements and molecules of compounds. He probably used a circle for each because, like the ancient Greeks, he thought of atoms as tiny, round hard spheres.