Elements

Elements Defined

Any material that is composed of only one type of atom is called a chemical element, a basic element, or just an element. Any material that is composed of more than one type of atom is called a compound. Every element has a unique atomic structure. Scientists know of only about 109 basic elements at this time. (This number has a habit of changing.) All matter is composed of combinations of one or more of these elements. Ninety-one of these basic elements occur naturally on or in the Earth. The other elements are man-made. You may recognize the names of some of these basic elements, such as: hydrogen, helium, oxygen, iron, copper, gold, aluminum, uranium. The periodic table of elements (shown below) lists the basic elements and some of their properties.

The periodic table is organized based on the atomic number of each element, which is located in the top left corner of each element box in the table below. The atomic number is the number of protons in the nucleus of an atom of that element. The atomic mass is also listed for each element below under its name. 

The periodic table also tells us about the state of matter each element is in at room temperature. While most elements are solids at room temperture, 11 elements, including Hydrogen, Helium, and Oxygen, are gases at room temperature and 2 are liquids.