What is the fundamental difference between an element and a compound?

An element is defined as a pure substance that consists of only one type of atom. This means that all the particles in an element are identical in terms of their atomic structure. The elements represent the simplest form of matter that cannot be broken down into simpler substances through chemical reactions. For example, oxygen (O), hydrogen (H), and gold (Au) are all elements, and each is made up of atoms that are the same.

In contrast, compounds are formed when two or more elements chemically combine in fixed proportions. These compounds have different properties from the individual elements that compose them and cannot be separated into their constituent elements by physical means, although they can be broken down into simpler substances via chemical reactions. The defining aspect of an element lies in its homogeneity—composed of just one atom type—making it distinct from compounds and mixtures.