Elements combine to form
compounds
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Compounds have different properties from
the elements that make them.
Compounds are substances made of atoms
of two or more different elements.
The atoms of different elements are held
together in compounds by chemical bonds.
The properties of a compound depend on
the bonds and how the atoms are arranged.
Elements combine to form
compounds continued…
Compounds have a specific ratio of
elements.
Ex: H2O
* Two atoms of Hydrogen for every
one atom of Oxygen
* Ratio is 2:1
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Chemical Formulas
An expression that shows the
number and types of atoms joined
in a compound.
 Subscript numbers are used with
symbol of element to show how
many atoms of that element are
present.
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Chemical Formulas
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A subscript is a number written to the
right of a chemical symbol and slightly
below it.
Ex: CO2
The chemical formula shows one
carbon atom bonded to two
oxygen atoms.
Same Elements, Different
Compounds
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Different compounds can be made
using the same elements, but in
different ratios.
Ex: H2O, ratio 2:1, hydrogen to oxygen
H2O2, ratio 2:2, hydrogen to oxygen
To sum up…
Each symbol in a formula represents an
element.
- If only one atom of an element is present
in the compound, no subscript is used.
- If more than one atom of an element is
used, then the symbol is followed by a
number indicating how many atoms are
used. This is called the subscript.
Another example…