What distinguishes an element from other substances?

An element is distinguished from other substances primarily because it is composed of only one type of atom. This means that all the atoms in an element share the same number of protons in their nuclei, which defines the element's identity on the periodic table. For instance, all the atoms of carbon are identical and contain six protons, while all atoms of oxygen contain eight protons. This characteristic of elements defines their purity and uniqueness.

In contrast, substances that can be separated into simpler substances consist of more than one type of atom or molecule, such as compounds or mixtures. The ability to undergo chemical changes easily does not uniquely identify an element, as many compounds and mixtures can also undergo chemical transformations. The statement about lacking physical form is incorrect because elements, like all substances, have mass and occupy space; they can exist as solids, liquids, or gases. Understanding that an element is a pure substance made up solely of one type of atom is fundamental to distinguishing it from mixtures and compounds.