What must a pure element consist of?

A pure element is defined as a substance that consists entirely of one type of atom. This uniqueness is what distinguishes pure elements from compounds and mixtures. In a pure element, all atoms share the same atomic number and chemical properties, which is fundamental to the identity of the element.

For example, gold (Au), oxygen (O), and hydrogen (H) are all pure elements, each composed solely of a single kind of atom. This consistency allows pure elements to maintain specific characteristics, such as melting and boiling points, reactivity, and atomic structure.

In contrast, a scenario involving multiple types of atoms would constitute a compound, such as water (H₂O), which contains both hydrogen and oxygen atoms. Mixtures, as opposed to pure elements, comprise two or more substances (elements or compounds) that retain their individual properties and can be physically separated.

Thus, the concept of a pure element resting on the presence of only one type of atom is foundational to understanding chemical substances and their classifications in chemistry.