The Atom: From Philosophical Idea to Scientific Theory

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Chapter 3.I

• The Atom From Philosophical Idea to Scientific
  Theory

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400 B.C. Greek Philosopher Democritus
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Democritus

• called natures basic particle the atom
• Atom was Greek for indivisible
• Believed that all matter was composed of
  individual atoms
• Greeks valued thinking/philosophy
• No scientific research was done to back up his
  theory

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Chemists of the 1700s

• Believed atom could not be broken down
• Elements combine to form compounds with different
  properties than those that they started with
• No understanding of how atoms came together to
  form compounds

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Antoine Lavoisier

• Developed a balance that for the first time could
  accurately measure mass.
• Recall that mass is the measure of the amount of
  atoms in a substance.

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3 Quantitative Laws of the 1800s

• Law of Conservation of mass
• Law of Definite Proportions
• Law of Multiple Proportions

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Law of Conservation of mass

• Matter cannot be created (made from nothing)
• Matter cannot be destroyed (be made into nothing)
• Particles simply rearrange to form new compounds
• Figure 2 p.69

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Law of Definite Proportions

• Chemical compounds contain the same elements in
  the same proportions by mass
• Does not matter how much sample there is
•  Ex. NaCl always is made up of 39 Na and 61Cl
  by mass
• The ratio is intrinsic it is the same whether
  you have 1 gram of NaCl or 100 g of NaCl

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Law of Multiple Proportions

• If two or more compounds are composed of the same
  elements (Ex CO and CO2) then the elements
  always come together in a ratio of small whole
  numbers
• Never CO2.5

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1800 England Teacher John Dalton
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John Dalton

• Penned the Atomic Theory
• Atomic Theory explained the three quantitative
  laws

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Atomic Theory

• All matter is composed of atoms
• 2. Atoms of the same element are identical same
  size, same mass, same properties

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Atomic Theory

• 3. Atoms cannot be subdivided, created, or
  destroyed (law of conservation of matter)
• 4. Atoms combine in simple whole number ratios to
  make compounds.
• (Law of definite proportions)
• (Law of multiple proportions)

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Atomic Theory

• 5. In chemical reactions, atoms are combined,
  separated, or rearranged.
• (Law of Conservation of Mass)

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Modern Atomic Theory

• Correction to point 2 Atoms of the same element
  can have different masses
• Two atom of the same element but with different
  masses are termed isotopes.
• Correction to point 3 Now we know atoms CAN be
  subdivided through nuclear reactions

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Daltons Model of the Atom

• Description

• Small
• Indivisible
• Identical for an element