A pure substance that cannot be broken down is an element. Atoms of the same element are identical physically and chemically. An element's atoms are physically and chemically different from those of another element.
Each element is represented by a specific symbol in the Periodic Table. Examples of elements are oxygen (symbolized by O), aluminum (symbolized by Al) and Argon (symbolized by Ar). About 90 elements on the Periodic Table can be found naturally on Earth. Three of the most common ones are aluminum, silicon and oxygen. Living organisms have hydrogen, carbon, sulfur, nitrogen, phosphorus and oxygen in abundance.
When two or more elements are chemically bonded, they form a compound. Some examples of compounds are water (symbolized as H2O), carbon dioxide (symbolized as CO2) and sodium chloride or table salt (symbolized as NaCl).