Individual atoms are too small to see, but packed together they make up familiar chemical elements.
Though hydrogen is the simplest of all atoms, it fuels stars like our sun and is vital for life. An atom of hydrogen has one proton in its nucleus and one electron moving around that nucleus. Other chemical elements, such as carbon, oxygen, gold, and mercury, are made of atoms with many electrons moving around a nucleus of many protons and neutrons. Some 450 years ago, only 12 chemical elements were known. As more were discovered, scientists noticed a natural order to them.