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As shown in the figure, different sets of elements are found in metal, polymer and
ceramic samples. The elements for metallic and polymeric materials do not overlap at all. This
is one of the reasons that the properties of those two classes are so different.
The elements found
in ceramics are also in found metals or polymers.
Compounds
A compound has a uniform composition: every sample removed and tested, no matter
how small, contains same elements in the same proportions. The elements in compounds are held
together tightly and can only be separated from each other by chemical reactions.
A chemical formula relays the chemical composition of a compound in a compact
manner. It includes the chemical symbol for each element, listed from left to right as they appear
on the Periodic Table. The symbol for each element is immediately followed with the number of
atoms of that element as a subscript. Fe
2
O
3
contains two iron atoms and three oxygen atoms. If
there is only one atom of the element, we do not bother to write the number one.
Water has two
hydrogens and one oxygen, so its chemical formula is written H
2
O.
The elements hydrogen and oxygen, and the compound water.
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Table salt is NaCl. Sand always contains one silicon for every two oxygens, so its formula is
SiO
2
.
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