Which of the following best describes an element?

An element is best described as a pure substance containing only one type of atom. This definition reflects the fundamental nature of elements in chemistry, where each element is defined by the number of protons in its atomic nucleus, known as the atomic number. Elements are the simplest form of matter and cannot be broken down into simpler substances through chemical reactions.

For example, oxygen (O), carbon (C), and gold (Au) are all elements comprised of a single type of atom. This property distinguishes them from compounds, which are formed when two or more different types of atoms bond together in specific proportions, and mixtures, which can consist of multiple substances that retain their individual properties. In summary, option B accurately identifies elements as unique substances represented solely by one type of atom, underscoring their foundational role in the study of chemistry.