[crazydictator]

I got a question that asked me what atoms could substitue for one another. Do I use a periodic table to make sure they are from the same class? or is there an easier way?
Thanks

[sjb]

It all depends what you mean by "match".

At a simple level, yes, atoms in the same group (like F, Cl, Br, I) would be a good example.

Once you get into biological systems and drug molecules, things get more complex and you can find that e.g. thiophenes and benzenes can often be exchanged with varying degrees of success, or things like secondary amines HNR¹R² and ethers R¹OR².

See for instance http://en.wikipedia.org/w/index.php?title=Isostere&oldid=110630228

S