In chemistry, an element is a substance that is unable to be broken down into smaller components by non- nuclear chemical reactions. An atom, on the other hand, is the smallest unit of matter. Every solid, liquid, and gas is composed of atoms. The chief difference between atoms and elements is that elements are a species of atoms that share the same chemical and physical properties. The number of protons and neutrons defines an element. An atom, on the other hand, is the smallest repeating unit of matter. Atoms that have the same quantity of protons in their atomic nucleus belong to the same chemical species.