Chapter 5
Early Atomic Theory and Structure

Early Thoughts of the Atom.

Empedocles (440 BC): Earth, Air, Fire, and Water.

Democritus (400 BC): Matter is composed of indivisible particles called "atoms." Unfortunately, he had no evidence for his views, and the influential Aristotle disagreed, saying that matter is continuous, so the "atomic theory" was disregarded until 20 centuries later.

 

Experimental Evidence for Atoms.

• Conservation of Mass: Lavoisier used precise measurements of mass in a closed system to show that mass is conserved when mercury (II) oxide is heated to produce metallic mercury and gaseous oxygen.
• Constant Composition: Compounds have the same composition, no matter how they are formed. For example, carbon dioxide has the same ratio of carbon and oxygen whether it is formed from the combustion of coal or the respiration of a guinea pig.
• Definite Proportions: Elements always combine in the same proportions. Look in your notes for the lead (II) sulfide example.
• Multiple proportions: atoms combine in small whole number ratios to form compounds. Different ratios yield different compounds.

Electric Charges

May be positive or negative.

Unlike charges attract, like charges repel.

Charge may be transferred from one object to another.

Less distance -> greater force.

John Dalton's Atomic Theory (1850).

1. Elements are composed of atoms.
2. Atoms are indivisible.
3. Atoms of the same element are identical, and unlike the atoms of any other element.
4. Atoms combine to form compounds.
5. Chemical reactions involve rearrangements of atoms to form new compounds.

Which of the above parts of Dalton's Atomic Theory are known today to be inaccurate.

 

Electricity Experiments

• Electrolysis of water.
• Michael Faraday - Charged atoms carry electric current.
• Svante Arrhenius - An "ion" is an atom which carries a charge.
• William Crookes - Invented the cathode ray tube, which was used to discover electrons and protons. A perforated cathode helped in the discovery of positive rays moving in a direction opposite to the cathode rays.
• Rutherford - Gold foil experiment. This is the one that led people to postulate that the atom has a nucleus, where all of the mass and positive charge resides.

Protons, neutrons, and electrons.

Atomic symbols.

Isotopes.

Abundance and average atomic mass.

Homework for Chapter 5

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Return to the Schedule.
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