If you do end up emailing head of chem/head of year, I'd make a point of including specific reasons/events that make you feel such a way, e.g. teacher screams at students during class regularly, missing out topics and/or leaving them to self study where other teachers haven't, makes you feel less than for asking questions or generally addressing you/others in an abrasive manner ("How come you got this question wrong, are you an idiot? I just went over this!" etc.)
Apologies if that seems obvious, but if you've already been in arguments with these staff, I think your best point is to approach them with as much objective evidence as possible towards this rather than just relying on your word that it's harming your mental health, as they might try to interpret it as you wanting to move classes to be with more friends or any similar reason that isn't so viable and/or turn it down just because they dislike you.
Might also be worth having your parents fight it out for you if it doesn't work out. Also, if you can get testimony from parents/GP/counsellors etc. that say that your mental health did seriously plummet as a result of chemistry in Y12, that might help.
Best of luck.