Answer :

Well none since molecules are a group of two or more atoms electrically bonded with one another. However, there are gases that does not naturally bond due to their stability and can be found in nature as pure elements. But these are not considered as molecules.

(By the way, these gases are the noble gases that can be found on the last column of the periodic table) 
Edufirst

Answer: the molecules formed by one kind of atom are pure elements


Explanation:


1) The definition of molecules is the covalent bonding of two or more atoms.


2) Under that definition you must have at least two atoms to be considered a molecule.


3) Pure elements are the 118 elements that you find in the periodic table: each contain only one kind of atom.


4) Then, molecules of pure elements, are two or more atoms of the same kind linked by covalent bonds.


5) With that these some examples of molecules that are pure elements:


i) O₂

ii) N₂

iii) F₂

iv) Cl₂

v) Br₂

vi) O₃


6) And here some examples of molecules that are not pure elements (there are many, many):


i) CH₄

ii) CH₃CH₂CH₃

ii) CH3CH₂CH₂CH₂CH₂CH₂CH₂CH₃


7) Note that ionic compounds are excluded of the definition of molecules, since they are not formed by covalent bonds but by ionic bonds. For example, NaCl is not a molecule.