I would like to know; since oxygen exists as a diatomic molecule, how is it that HCl doesn't?
thank you!

Its explanation is based on chemical bonding .Complete explanation can not be written here .So I request you to read chapter chemical bonding carefully and you can ask questions from that chapter.I will help you to understand it. 

That's the problem, the text that we use doesn't have anything on chemical bonding and we never did it. My teacher doesn't refer to that text, she has another one which we don't have because it wasn't in our book list.
Is there any way that you can help me out?? (i am a quick learner)
thank you!!

sure
I can give you a good presentation on chemical bonding.
Is the email id provided by you here in the forum is active ?

yes.
thank you soooooooo much miss!!!!!!!!!!