It probably depends on the quantity of chlorine and fluorine present. However, even in small quantities, chlorine gas is highly toxic and for this reason is routinely used as an antimicrobial agent in municipal drinking water treatment. Both chlorine and fluorine are highly electronegative elements and can rapidly destabilize organic molecules. For this reason, it does not seem likely that LIFE AS WE KNOW IT could evolve under such conditions you describe (a fluorine and chlorine-rich atmosphere).

Life as we know it has never developed in a chlorine or fluorine atmosphere, as you know.I expect the main reason is that there aren't any places on Earth with chlorine or fluorine atmospheres. I just read about a kind of bacteria that has an enzyme (do you know this word?) that can put fluorine onto molecules - scientists had never known before that this was possible. The news article was joking that maybe the little green men on Mars are looking at Earth and saying that there can't possibly be life on Earth because there's too much oxygen, and oxygen attacks even metals! So I'll not worry my brain thinking about the chemistry of fluorine or chlorine too much, since we (scientists) are continually discovering life in places where we thought it would be impossible to live - very deep in the ocean where the pressures are huge and there's no sunlight, for example.

I do know, though, that oxygen has an advantage over fluorine or chlorine in that an oxygen atom can bond to 2 different atoms as in water, H₂O, while chlorine and fluorine can only bond to 1 other atom, as in HCl, or carbon-containing molecules with chlorine or fluorine attached. So oxygen can be a bridge between 2 atoms and not just hang on to another atom. Also, some bacteria can live in water with lots of sulfur, and sulfur is like oxygen in being able to attach to 2 other atoms. So - I expect life could develop in an atmosphere with quite a bit of chlorine or fluorine, and I don't know if it could develop in an atmosphere that was mostly chlorine or fluorine, or not.

Keep up with your science questioning, and check out www.google.com to search out answers.