They are not always interchangeable. If is used much more often, because it's more flexible. It fits in more situations. Whether needs to be followed by a choice of options. If can precede a choice or a single statement. Examples: OK: if you ask me, I will tell you. Not OK: whether you ask me, I will tell you. But OK: whether you ask me or not, I will tell you. Also OK: if you ask me or not, I will tell you. OK: I will go if he does. Not OK: I will go whether he goes. But OK: I will go whether he goes or she goes. Also OK: I will go if he goes or she goes.

According to my English teacher, whether and if are the same :)