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, Aristotle disagreed,
saying that matter is continuous, so the "atomic theory" was
disregarded until 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).
- Elements are composed of atoms.
- Atoms are indivisible.
- Atoms of the same element are identical, and unlike the
atoms of any other element.
- Atoms combine to form compounds.
- 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.
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