All matter is considered to be composed of unit substances known as chemical elements .
These are the smallest units that are distinguishable on the basis of their chemical activity and physical properties . The elements are composed of atoms which have distinct structure characteristic of each element. A substance that can be broken down into two or more simpler substances. The smallest part of a substance that retains the same properties of that substance and can not be broken down any further. Elements are composed of atoms which have a distinct structure characteristic of each element.