CHEMISTRY TIMELINE #11800'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

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.”

• 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.

(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”

Development of Dalton’s Theory

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.•

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 = 0.75• • Chemists used proportion problems like those above until all of the elements were

assigned a relative mass.

Development of Dalton’s Theory

(Dalton, 1803)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 wholenumbers”

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.

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

Law of Multiple Proportions: (Cont.)

• When oxygen combines with nitrogen it does so to produce three different ratios by mass.

• Compound A = 1.7500g of N for every g of O.• Compound B = 0.8750g of N for every g of O.• Compound C = 0.4375g 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 NO2 N2O2

• compound C NO2 N2O2 N2O4

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

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

Dalton’s Atomic Theory

• 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.