Question

What gases in Earth's atmosphere would you expect to find as molecules and which as individual atoms?

Answer 1

The only somewhat significant gases that occur as pure elements will be `"Ar"` and trace amounts of `"Ne"` and `"He"`.

All other gases that contribute a significant amount to the atmosphere's composition will be molecules.

Explanation:

Well, recall that there are seven elements that occur naturally diatomic, which means they occur in nature as two-atom molecules. These seven are

• `"H"_2`

• `"N"_2`

• `"O"_2`

• `"F"_2`

• `"Cl"_2`

• `"Br"_2`

• `"I"_2`

Of these seven, the first five are gaseous at standard conditions, and of those five, there are only two that exist to a significant extent in Earth's atmosphere:

• `"N"_2` (`78.0%`)

• `"O"_2` (`21.0%`)

The remaining `1%` is taken up mostly by argon, `"Ar"` (`0.9%`), and the rest are trace gases, such as `"CO"_2` (`0.093%`), `"Ne"` (monatomic, `0.0047%`), `"He"` (monatomic, `0.0013%`), and others such as `"CH"_4` (methane), `"N"_2"O"` (nitrous oxide), and `"O"_3` (ozone).

Below is a neat little pie chart of gaseous abundance in Earth's atmosphere: