What is the main difference between an element and a compound?

The main difference between an element and a compound lies in their composition and the fundamental substances involved. An element is defined as a pure substance that cannot be broken down into simpler substances through chemical means. It consists of only one type of atom and represents the basic building blocks of matter. For instance, oxygen and gold are both examples of elements.

On the other hand, a compound is formed when two or more different elements chemically combine in fixed proportions. This means that compounds consist of multiple types of atoms bonded together, resulting in new substances with properties different from those of the individual elements. Water (H2O) and sodium chloride (NaCl) are examples of compounds.

The other options contain misconceptions. For example, the statement that an element has a fixed composition while a compound does not is incorrect because both have fixed compositions—elements are uniform in atomic type, whereas compounds have specific ratios of their constituent elements. The notion that elements are always naturally occurring while compounds are synthetic overlooks many natural compounds, and the idea that an element is a mixture contrasts with the definition of an element as a pure substance. Thus, the distinction made in the chosen answer encapsulates the essence of how elements and compounds differ.