Which of the following best describes an element?

An element is defined as a pure substance that is made up of only one type of atom. This means that all atoms in an element share the same number of protons in their nuclei, which is what determines the element's identity and properties. For example, hydrogen is an element that consists solely of hydrogen atoms, while oxygen is made up entirely of oxygen atoms.

In contrast, a complex mixture of various atoms does not meet the criteria for being an element since it contains multiple types of atoms. A type of bond between molecules refers to interactions that can occur between different substances or elements, but does not define what an element is. Lastly, describing an element as having an unstable atomic structure is misleading, as elements can possess stable isotopes, which are variations of the element with different numbers of neutrons. Thus, the option that emphasizes a pure substance consisting of only one type of atom accurately captures the essence of what an element is.