Elements, Compounds and Molecules

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Elements, Compounds and Molecules

• Section 5.3

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• In total, 117 elements have been observed as of
  2008, of which 92 occur naturally on Earth the
  others are man-made!
• Every kind of matter has to be made up of one or
  more of these 117 elements.

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• Matter that is made up of only one type of atom
  is called an element (like those on the periodic
  table)
• The atoms of any one element are exactly alike
  but the atoms of any given element are different
  from atoms of every other element
• Example
• All hydrogen atoms are the same as all other
  hydrogen atoms but they are not the same as
  oxygen atoms

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• Most of the substances we know are made up of two
  or more elements
• Water (H2O), salt (NaCl), carbon dioxide, (CO2)
  and baking soda (NaHCO3)
• These substances are possible because atoms of
  different elements can link-up
• Elements are pure substances that cannot be
  broken down into simpler parts by ordinary
  chemical means
• Elements combine to produce compounds

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• Some common elements are gold (Au), copper (Cu),
  hydrogen (H), oxygen (O)
• Most elements are found in a solid state but they
  can also be gasses and liquids (there are only 2
  liquids)

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• A compound is a substance made up of two or more
  elements
• When elements combine to make compounds they lose
  their own properties the new compound has its
  own properties
• Compounds dont have to be in the same state as
  the elements from which they are made
• Example
• hydrogen (gas) and oxygen (gas) make water
  (liquid)

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Molecules

• A molecule is the smallest part of a compound
  that still has the properties of the compound
• It must have at least two atoms but can have
  thousands.

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Atomic Theory

• Explaining Chemical Facts and Laws

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Daltons Atomic Theory

• All matter is made up of small particles called
  atoms
• Atoms cannot be created, destroyed or divided
  into smaller particles
• All atoms of the same element are identical in
  mass and size, but they are different in mass and
  size from the atoms of other elements
• Compounds are created when atoms of different
  elements link together in definite proportions

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• Daltons model uses the idea that elements are
  different because their particles are different

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Laws vs. Theories

• In science
• Laws do not explain anything, they just describe
  and summarize
• Theories explain why something happens

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Atomic Masses

• Atomic mass is the mass of an average atom of an
  element
• Hydrogen was the lightest known element at the
  time so Dalton gave it an atomic mass of 1

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• Since the atomic mass of Hydrogen was 1 Dalton
  theorized that all other atomic masses could be
  compared to the mass of Hydrogen.
• For example, the mass of Lithium is 3 so it is 3
  times larger than Hydrogen etc.

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• In Daltons New System of Chemical Philosophy,
  published in 1808, Dalton provided relative
  masses for 20 elements to support his atomic
  theory.
• He also showed 17 examples of how the theory
  explained the formation of compounds by proving
  that H2O has an atomic mass of 10