What distinguishes an element from a compound?

An element is defined as a pure substance that is made up of only one type of atom. This means that every atom in that element shares the same atomic number, which determines its identity and properties. For example, oxygen is an element composed solely of oxygen atoms.

In contrast, a compound is a substance formed when two or more different types of atoms bond together chemically, resulting in a new substance with distinct properties that are different from the individual elements that comprise it. This key difference highlights that compounds are made from different types of atoms, rather than just one type, which defines elements.

The other choices do not accurately represent the distinction between elements and compounds. One asserts that a compound consists of a single type of atom, which is incorrect, as compounds by definition must contain multiple types of atoms. Another incorrectly categorizes elements and compounds based on their physical states, which can vary significantly. Lastly, the statement regarding variable compositions does not accurately characterize elements, as each element has a defined and fixed atomic composition.