Question

Why is ${O_2}$ a molecule?

Answer 1

Pure oxygen atom is found to be highly reactive. It will combine with any of the surrounding atoms to produce oxides. Since it is in a nascent state it is highly prone to form bonds. It is available as ${O_2}$ and not as O. If no elements are present to bind with oxygen it will bond with itself to produce the ${O_2}$ molecule.

Complete answer:
A molecule is defined as the cluster of atoms having two or more atoms. The atoms can be of the same type or different types. The molecules like ${O_2},{H_2}$ etc. that have same two atoms are known as homonuclear molecules (homo means same), whereas the molecules such as $HCl,{H_2}O$ etc have two different molecules and they are known as heteronuclear molecules. Both these types of molecules come under the category of molecules itself.
A molecule can be simple or can also be complex. When there are a no. of different elements present in the molecule it is known as a compound. A molecule is formed if two similar or dissimilar atoms are bonded together by chemical bonds.
Consider the following examples:
${O_2}$ - simple homonuclear molecule having two atoms of oxygen
${H_2}O$ - simple compound that has three atoms; two of hydrogen and one of oxygen.
${C_6}{H_{12}}{O_6}$ - a complex compound (glucose) formed between Carbon, Hydrogen and Oxygen.

Note:
Although we say that the simplest form of a pure substance is an element, many elements do not occur in their free state in nature. Consider oxygen, it doesn’t occur as O and always occurs as ${O_2}$ in nature. When we consider an element, we always consider their atomic forms, but many of the gases like ${H_2},{N_2}$ occur in their molecular forms only, and that is fine.