Introduction

Our universe composed of 90 elements that cannot exist alone. They are present in arrangement: diatomic compounds. According to Avogadro’s hypothesis, “under the same condition, equal volume of gases contains the same number of molecules.” Example: He and H2. Hydrogen composed of two atoms because in mono-atomic state it is not stable. The connection between two atoms called “chemical bonding”. However, inert gases, such as He, Ne, Ar…, can exist alone. They called inert gases.

Source:

http://serc.carleton.edu/usingdata/nasaimages/index4.html

Which molecules are mono-atomic?