In "whether" you have two or more possibilities, and the conclusion is often expected to be true even if neither possibility is true. It is also normally expected that one (and only one) of the possibilities will come true. In this one you must normally state "or not" when that is one of the possibilities. "Whether you have a boy or a girl, you should start a college fund." This statement expects that you will have a child, and that you will start a college fund regardless of its sex. In "if" you have one or more possibilities and, normally, the possibility that the "if" is not true, whether that is stated or not. The conclusion is only assumed to be true when the preceding "if" portion is true. Otherwise, the conclusion's accuracy is unknown. "If you have a boy or a girl, you should start a college fund." This statement doesn't know for sure that you are even having a child. You should only start a college fund in the case that you have one.

Hola http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv179.shtml