An example might clear things up.
If a
carbon atom has four different groups attached to it, then there are two different ways in which these groups can be arranged around the carbon
atom. Such a carbon atom is known as an
asymmetric carbon atom. The two different forms of the
molecule are known as enantiomers. They are mirror images of each other but cannot be superimposed on each other. The molecules below are both
alanine (2-aminopropanoic acid), yet they possess
unique bonding arrangements.
NH2 NH2
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CH3---C---COOH CH3---C---H
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H COOH