I would assume that the atmosphere would probably consist of largely nitrogen, since it's such an innert gas, and then oxygen or something just as
corrosive corresponding on the periodic table.
Bold words, now I have to back them up
Our atmosphere used to consist largely of nitrogen and carbon dioxide. If you'll note, the nitrogen levels theoretically have not changed, as is
apparent through archiology, rocks, etc. Not many life forms, if any, are nitrogen dependant here on earth. But they were carbon dioxide dependant.
Life here on earth started pretty slowly, with just some random bacteria doing their thing. Slowly the bacteria started to form simplistic multicelled
organisms which were content just to exist. Then something happened.
A single celled organism advanced into algae. Now life was in trouble. It was fairly content to stay as it was, dependant on the 28% carbon dioxide
atmosphere of earth. But the algae were doing something bad. They were processing the carbon dioxide into O2. Pure oxygen. Oxygen is an extreamly
corrosive gas, and most of the multi-celled lifeforms couldn't take this, and started to die.
However, the oxygen had a very unexpected side effect. The organisms which didn't die adapted. And quickly (in evolutionary terms). Compared to the
time it took the original single celled guys to form multi celled organisms, it was a mere second before these oxygen adapted life forms developed the
multi celled lifeforms. They advanced at an astonishing rate. It wasn't long before plankton was roaming the seas.
But the algae still flourished, they could utilize a smaller amount of carbon dioxide than the multi celled creatures, and continued, even though they
had depeleted the carbon dioxide levels to about 14% of the atmosphere. And the plankton were helping. When they processed the oxygen, they would
"exhale" CO2, maintaining a balance in the atmosphere.
A synergy had been found, and life began to flourish. It wasn't long before we had fish, then triassic dinosaurs, and eventually mammals.
Intelligence capable of utilizing it's intelligence has only been apparent in the creatures which inhaled this volitile gas, oxygen.
Thus, in a way, oxygen spawned intelligence capable of acting on it's intelligence. (I wish oxygen would teach me to spell
) And all this time, the
nitrogen stayed the same.
So in summary, I think there would be an oxygen atmosphere, because the next step would be sulfur, which isn't a gas, at least at typical
temperatures. At the temps it would take to turn sulfur to a gas, you would have to be a little bit closer than Mercury (the planet, not the element)
to the sun, which would strip the atmosphere away, anyway. And just forget selenium.
As for the nitrogen, we have the same problem. The next step would be postassium, which would be stripped from the atmosphere before becoming a gas.
It's possible that a noble gas could make up the majority of the atmosphere, but life here finds those to be toxic. Something pre "8" on the table
excluding lithium could possible be used, but I don't know, because I don't know much about their chemical properties.
Sorry for being so long winded, I've done a lot of research into xenobiology.