Question #b2c22

Apr 24, 2017

The monatomic molecule here is $H e$, a noble gas.
Diatomic molecules are $O 2 \mathmr{and} N 2$; both highly reactive gasses.
Polyatomic molecules are $O 3 \mathmr{and} S 8$ an allotrope of sulfur.

Explanation:

Noble gases like $H e$ are very stable elements forming molecules of only one atom. The reason is their complete outer shell of electrons, so they do not need to add, subtract, or share valence electrons with other atoms to be complete.

Elements that do need an extra electron to complete their outer shells include $O \mathmr{and} N$. Both of these gases have only seven electrons in their outer shells and so react vigorously with other elements that can supply the missing electron.

If there are no other types of atoms present to react with, the atoms will react with each other to share an electron to complete their shells and become more stable as a diatomic molecule.

Polyatomic molecules occur as the result of special reactions and are often allotropes of a substance. $O 3$ is a allotrope of $O 2$, and forms in the atmosphere on exposure to ultraviolet light or lightening. $S 8$ is naturally produced in volcanic activity, and is mined in areas around volcanoes.