"If" is a more common word than "whether." "Whether" appears with the words " or not." Examples: I will need an umbrella if it rains. I will take an umbrella whether or not it rains. IF- conditional WHETHER OR NOT- unconditional.

"If" is a synonym of "whether", as you know. There are several other ways the word "if" is used. Therefore I prefer to use "whether" in place of "if" so as not confuse my reader unduly.