A Question on the Subject of Nuclear Technology

So, I'm in the middle of chemistry studying and I just so happen to have this question about something I'm reading.

According to this, isotopes chemically "behave in exactly the same way" with the only real difference between them being the mass. If this is so than why do you need more uranium-235 to create a nuclear bomb than is in natural uranium ?

I do not understand, how can that and the fact that they behave "exactly the same way" can both be true? This feels like that scene in Fiddler on the Roof--anyways--there has to be something more to this, don't you think?

Obviously he should have spent a little more time on this subject. I wish he'd spend more time on the stuff that I want to know and not the stuff I already know.