What defines an element in chemistry?

An element in chemistry is defined as a fundamental substance that cannot be broken down into simpler substances by ordinary chemical means. This definition highlights the unique and basic nature of elements, which serve as the building blocks of all matter. Elements are pure substances that consist entirely of one type of atom. For instance, elements like carbon, hydrogen, and oxygen each consist of their specific atoms and cannot be decomposed into other substances without altering their fundamental properties.

The other choices mischaracterize the definition of an element. An element is not something that can be broken down by chemical means (as stated in the first option), nor is it a compound, which is formed from the chemical combination of two or more different elements (as suggested in the third option). Additionally, an element is not a mixture of substances; mixtures contain multiple components that can often be separated back into their original substances and do not represent a single type of atom or molecule. This clarification reinforces the definition of an element as a singular, irreducible substance in chemistry.