What is the definition of an element in chemistry?

In chemistry, an element is defined as a pure substance that consists of only one type of atom. This means that every atom in an element has the same number of protons in its nucleus, which determines the element's identity and its position on the periodic table. For example, all carbon atoms have six protons, making carbon a distinct element. Elements cannot be broken down into simpler substances by chemical means.

The other choices describe properties or combinations of substances rather than elements themselves. For example, the first option refers to compounds or mixtures, the second option mentions mixtures rather than pure substances, and the fourth option directly pertains to compounds that consist of multiple elements rather than single ones. Understanding this foundational concept is crucial in chemistry, as it sets the stage for further exploration of compounds and reactions involving different elements.