What is the smallest unit a compound can be broken down into without altering its characteristics?

The correct answer is a molecule. A molecule is the smallest unit of a compound that retains all of its chemical properties. It consists of two or more atoms that are chemically bonded together. When you break down a molecule into its constituent atoms or parts, you may alter the characteristics and properties that define that compound. For example, water (H₂O) is a molecule composed of hydrogen and oxygen, and it exhibits unique properties such as boiling and freezing points. If water is broken down into its individual elements, hydrogen and oxygen, those elements cannot exhibit the same properties as water; they have different characteristics.

In contrast, an element is a pure substance that cannot be broken down into simpler substances by chemical means, and it does not inherently retain the properties of a compound. A compound refers to a substance formed when two or more elements are chemically bonded, but it is the molecule that specifically retains the properties unique to that compound. Lastly, a mixture consists of two or more substances that are not chemically bonded, and therefore, does not refer to a specific unit that retains compound characteristics.