Chemical elements are formed by a single type of atom (Na, K, O 2 , N 2 ), while compounds are formed by the union of two or more types of atoms. Water is a chemical compound made up of two hydrogen atoms and one oxygen (H 2 O). Carbon dioxide is made up of one carbon atom and two oxygen (CO 2 ). Chemical compounds are represented by the symbols of the elements that form them accompanied by subscripts that indicate the proportion in which these elements participate.