Structural isomers are chemical compounds that have the same atoms arranged in different orders.
The definition of isomers is two or more compounds that have the same molecular formula (same kind of atoms and same number of atoms of each kind) but have different molecular structures (the way in which the atoms are arranged).
There are different kind of isomers. Two of them are structural isomers and geometrical isomers.
Structural isomerism means that the atoms are bonded in different ways (orders). This causes that the compounds have different physical and chemicall properties, despite they have the same molecular formula.
An example of structural isomers is:
CH3 - CH2 - CH2 - CH2 - CH3 (pentane)
CH3
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CH3 - CH - CH2 - CH3 (2-methyl butane)
CH3
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CH3 - C - CH3 (2,2, dimethyl propane)
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CH3
You can count that the number of atoms of C and the number of atoms of H are the same in the three molecules. They differ in the order of the atoms and are different products.
