In traditional Western culture, addressing a person by his family name only is mostly done in a superior-to-inferior context (military, very formal work environment, strict and old-fashioned school environment) with the nuance that the superior uses it that way to assert his superiority (somewhat rude, but the inferior has to swallow his pride). As such I would say it would be considered rude, or at least ignorant, if a student addresses a professor like that.
Using a person's given name is considered more intimate or at least informal in Western culture, typically only used in cases like:
- family (only some relations, depending)
- colleagues, only in cases where the corporate culture is less formal or explicitly states it (but that is the case in the vast majority of companies nowadays)
- when preferred by a person (typically, the person would introduce him/herself with something like "Hi, I'm
firstname" or "Just call me
If none of these cases apply, it is safer to use a more formal form of address, until the other party invites you to use a less formal address. The more senior party would extend such an invitation (see last point above). If you think your are (fairly) equal to a new person you are meeting (e.g., a fellow student, or joining a company and meeting colleagues) it should be OK to invite them to address you by your given name.
The other point nobody has mentioned, is that if not (yet) on an informal level, it should always be OK to address the other person by their title (only). An American professor should not take exception to being addressed as "Hello, Professor!" when met off-campus.
In general life it should be OK to address a stranger by an assumed general title like "mister", "miss", "mrs" ("sir", "madam") etc., and the person should have the manners to correct you with good grace if he/she prefers a different title (e.g., "Oh, I'm professor Smith..."). Or a business card/credit card/letterhead etc. should give hints at the person's title, if such materials are available. (Due to feminism, "Miss" and "Mrs" are problematic, but the proper replacements for such is another topic.)