Which of the following is the smallest unit of a compound?

The correct answer is a molecule. A molecule is defined as the smallest unit of a compound that retains all of the chemical properties of that compound. It is formed when two or more atoms bond together chemically. For instance, water (H2O) is a molecule made up of two hydrogen atoms and one oxygen atom. When discussing compounds, it is key to note that a molecule retains the chemical characteristics of the substance, allowing it to participate in chemical reactions and exhibit specific properties.

Atoms, while they are the fundamental building blocks of matter, do not represent a compound as they can exist as single entities and do not necessarily possess the properties of a compound by themselves. An element refers to a substance that consists of only one type of atom and cannot be broken down into a simpler substance, whereas a mixture is a physical combination of two or more substances where each retains its own properties, rather than forming a new compound. Thus, a molecule encapsulates the essence of a compound more accurately than the other options.