What is a fundamental substance that cannot be broken down any further?

The fundamental substance that cannot be broken down any further is an element. Elements are pure substances consisting of only one type of atom and are the basic building blocks of matter. Each element is defined by its unique number of protons, which determines its position on the periodic table and its chemical properties.

In contrast, a compound is made up of two or more elements that are chemically combined in fixed ratios, meaning it can be broken down into its constituent elements. A molecule refers to two or more atoms bonded together, which can be either elements or compounds. Lastly, a mixture consists of two or more substances that are physically combined but not chemically bonded, allowing each substance to retain its own properties. Thus, while compounds, molecules, and mixtures can all be further divided or separated, elements stand alone as the most basic form of matter.