What do specific gases do?

[telegraph (OP)]

There are a lot of different gases to be added to an atmosphere.  What are the effects of each of those? For example how is helium is different from Neon?

[sloanjh]

There are a lot of different gases to be added to an atmosphere.  What are the effects of each of those? For example how is helium is different from Neon?

It's atomic number is smaller? 

As far as I know, there's no difference in the noble gasses.  You need the ability to adjust both, however, in case the pressure is too high due to one of them.

Just going from memory, the important gasses are:

Oxygen:  Breathing is nice
CO2: Greenhouse gas, plus I try to adjust it to ~2% when playing humans for role playing purposes (the game doesn't enforce it, but plants have to breath too )
Greenhouse/anti-greenhouse gasses: Used to adjust temperature
Toxic gasses: increase colony cost
Neutral gasses (Nitrogen, Helium, etc.):  Affect pressure
Methane:  Methane breathers prefer methane (although I don't remember if you can have methane breathers as a player race).

John

[o_O]

You can play as methane breathers, you just drop your empire down on a methane planet.  I heard there was a version where a species could breathe a couple other things too, but I don't know.   

Anyway, basically there is oxygen/methane that you breathe, greenhouse/antigreenhouse to control the temperature, toxic gasses that you need to destroy, and filler gasses to keep the oxygen % down to something reasonable.