Any substance consists of repetition of molecules. A molecule can be further divided into one or more atoms. An atom is the indivisible particle. However, the molecule is the one which tells us the physical and chemical properties of an element of compound. A molecule stays together (with all its atoms) whether the element is a gas, liquid or solid.
The number of electrons in an atom and their arrangement decides whether an element forms bonds or not.
For elements like Argon and helium, the molecule consists of only one atom. The atoms do not form bonds with neighboring atoms. They do not share electrons with neighbours, as their electronic configuration is saturated. They have electrons completely filling available slots. There is no space or requirement for attracting electrons from neighboring atoms. That is why they are called inert or noble gases.
There are many other elements that consist of one atom in one molecule, like carbon, Silicon, Calcium, Sodium etc.
For other gaseous elements like oxygen, chlorine and hydrogen, there are multiple atoms in a molecule. These elements have empty slots for electrons. So they try to attract or give away their electrons. So they interact with neighboring atoms.
A Molecule with more atoms than one is like a family having wife, husband, children. These persons form bonds among themselves.
A molecule with one atom in it is like a family in which there is one single man or woman. These persons do not (like to , or unable to ) form bonds with others. So they are single.