Final answer:
A Definition by Subclass is a classification method for substances based on composition and bond types, dividing them into atomic elements, molecular elements, molecular compounds, or ionic compounds. O2 is an example of a molecular element.
Step-by-step explanation:
Definition by Subclass is a method of classifying substances based on their composition and the types of bonds that hold them together. Substances can generally be classified into one of four categories: atomic element, molecular element, molecular compound, or ionic compound.
For example, let's consider the substance O2. It is made up of only one type of atom, oxygen, and these atoms are bonded to each other non-ionically. Therefore, O2 is classified as a molecular element because it consists of molecules made up of single type of atom joined by covalent bonds.