An element is defined as a substance that has what characteristic?

An element is defined as a substance that consists of atoms with the same atomic number. The atomic number is a fundamental characteristic of each element, as it represents the number of protons in the nucleus of an atom. This unique number determines the chemical properties of the element and its place on the periodic table.

Elements are categorized based on their atomic number, which remains constant for all atoms of a given element. While atoms of the same element can exhibit variations in the number of neutrons (leading to different isotopes), the defining characteristic that distinguishes one element from another is the atomic number, not the mass or the presence of neutrons. This is why an element is understood to consist of atoms that all share the same atomic number, making option C the correct choice.