Natural fatty acids commonly have a chain of four to 28 carbon atoma ( usually unbranched and even numbered ) , which may be saturated or unsaturated . In chemistry , especially biochemistry , a fatty acid is a carboxylic acid with a long unbranched aliphatic tail ( chain ) , which is either saturated or unsaturated . Most naturally occurring fatty acids have a chain of an even number of carbon atoms , from 4 to 28 .