Each element has a symbol listed on the periodic table of elements, which generally is an abbreviation of its name, (i.e. Al for Aluminum) but sometimes these symbols are derived from Latin words (i.e. Au for Gold.)
Chemical formulas for molecules composed of only one type of element can be either monatomic or diatomic.
Monatomic molecules consist of a single atom. The only monatomic molecules are those formed by the noble gases (the elements in the last column of the periodic table, helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and radon (Rn).
Diatomic molecules consist of two atoms of one element. There are five diatomic molecules that are gaseous at room temperature: Hydrogen: H2 , Oxygen: O2 , Nitrogen: N2 , Chlorine: Cl2 , and Fluorine: F2 . Bromine: Br2 is a diatomic liquid at room temperaure, and Iodine: I2 is a diatomic solid at room temperature.
In chemical formulas, subscript numbers represent the number of atoms of an element in a compound. Ones (1) are understood and not written, while all other numbers are. For example, the chemical formula for water is H2O , meaning 2 atoms of hydrogen and one atom of oxygen make up each single water molecule.