What are diatomic and atomic molecules?

Posted on 2016-03-03 18:40:00 in Chemistry by ROY

I am new to chemistry. I was doing a worksheet. And basically you had to correct the way it was written. I will show you what I mean (also please note the numbers I put are meant as subscript text). 

Br -- > the correct one: Br2, because it is diatomic. 

O --> O2, because it is diatomic. 

Fe2 --> Fe, because it is atomic. 

So I sort of understand that diatomic means they have 2 molecules, hence the 2. But I am wondering then, does that mean an oxygen molecule is NEVER on its own? Like just O? And then also how is Fe2 wrong? Can you never have 2 like that, or any number? I'm quite confused.. 

thanks.

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