Transcript What do you know about light?

Interpreting Chemical Formulas
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• Atoms!!
• http://www.brainpop.com/science/matterandch
emistry/atoms/preview.weml
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• A Chemical Formula uses symbols and
numbers to represent the composition of a pure
substance.
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• The chemical formula for water represents:
– The composition of water wherever it is found.
The chemical formula for water is always the
same!
– The composition of a molecule of water. The
chemical formula tells us that a water molecule s
made up of 3 atoms; 2 hydrogen and 1 oxygen.
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Symbol for the
element
hydrogen
The small
number here
means 2 atoms
of hydrogen.
Symbol for
the element
Oxygen
No small
number
means 1
atom of
oxygen
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• The atomic number of an element provides
information about its atomic structure.
• For example, the atomic number of fluorine is
9, indicating that there must be 9 protons in the
nucleus.
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• On its own the atomic number does not tell us
the number of neutrons in an element. In order
to determine the number of neutrons we need
the mass number.
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• The proper notation for writing about the
structure of an atom uses both the mass
number and the atomic number.
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• The atomic masses in the periodic table are
stated in atomic mass units (u).
An ion is an atom, or group of atoms, that has a net
positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons