A fatty acid having of a hydrocarbon chain and a terminal carboxylic acid group that is shown in the figure.  Most  fatty  acids  are  in  biology  have  an  even  number  of carbon  atoms  arranged  in an unbranched  chain.  Chain length commonly ranges from 14 to 24 carbon atoms with the most common fatty acids containing 16 or 18 carbon atoms. The saturated fatty acid has all of the carbon atoms in its chain saturated with hydrogen atoms. This gives the common formula CH3 (CH2)nCOOH, where  n  is  an  even  number.  The Monounsaturated fatty acids have one double bond in their structure although polyunsaturated.

Figure:  Structures of (a) a saturated fatty acid (b) a monounsaturated fatty acid with the double bond in the cis configuration (c) a monounsaturated fatty acid with the double bond in the trans configuration and (d) a polyunsaturated fatty acid