What is an element?

An element is defined as a pure substance that consists exclusively of one type of atom. This means that the characteristics and properties of the element—the way it behaves chemically and physically—are all derived from that single atomic species. For example, the element oxygen is comprised entirely of oxygen atoms, and it cannot be broken down into simpler substances by any chemical means.

In contrast, compounds are substances that are made up of two or more different types of elements that are chemically bonded together, which would align with the first choice. The third option refers to compounds or mixtures that can be separated into their constituents through chemical reactions, while the last option describes mixtures, which are combinations of different kinds of atoms or molecules that are not chemically bonded. Thus, the definition of an element is specifically about purity and the singular nature of the atoms that compose it.