What is defined as two or more atoms joined together?

The term for two or more atoms that are chemically bonded together is "molecule." Molecules can consist of the same type of atom, such as in molecular oxygen (O2), or different types of atoms, as seen in water (H2O). In essence, molecules represent the smallest fundamental unit of a chemical compound that retains the chemical properties of that compound.

When atoms bond together, they can form simple or complex structures, and this bonding is critical for the formation of the various substances that make up the physical world. Understanding molecules is foundational in chemistry, as they interact and undergo reactions to form new compounds, demonstrating the dynamic nature of matter.

Other terms in the options serve different purposes within chemistry. For example, a compound is a type of molecule that contains at least two different elements. An ion refers to an atom or molecule that has a net electrical charge due to the loss or gain of one or more electrons, while an electron itself is a subatomic particle with a negative charge and does not refer to a collection of atoms at all. Therefore, identifying "molecule" as the correct answer highlights the importance of understanding how atoms bond to create the building blocks of matter.