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

Sep 2, 2017

The only somewhat significant gases that occur as pure elements will be $\text{Ar}$ and trace amounts of $\text{Ne}$ and $\text{He}$.

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

• ${\text{H}}_{2}$

• ${\text{N}}_{2}$

• ${\text{O}}_{2}$

• ${\text{F}}_{2}$

• ${\text{Cl}}_{2}$

• ${\text{Br}}_{2}$

• ${\text{I}}_{2}$

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:

• ${\text{N}}_{2}$ (78.0%)

• ${\text{O}}_{2}$ (21.0%)

The remaining 1% is taken up mostly by argon, $\text{Ar}$ (0.9%), and the rest are trace gases, such as ${\text{CO}}_{2}$ (0.093%), $\text{Ne}$ (monatomic, 0.0047%), $\text{He}$ (monatomic, 0.0013%), and others such as ${\text{CH}}_{4}$ (methane), $\text{N"_2"O}$ (nitrous oxide), and ${\text{O}}_{3}$ (ozone).

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