Chemical formulas are a type of shorthand utilizing symbols and numbers to indicate the composition of a compound and the number of atoms of an element in one molecule of the substance.
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.
An empirical formula indicates the simplest, whole number ratio of the atoms in a compound. Sucrose, which exists in nature as C6H12O6, has an empirical formula of CH2O because H and O are in a one-to-one ratio, while H is in a two-to-one ratio with C and O.
A molecular formula shows the actual composition of the molecule in nature. For example, the subscripts of the empirical formula for sucrose are multiplied by 6 to get the molecular formula C6H12O6.
Important: Some compounds have the same empirical formula, so one must find the molecular formula to distinguish between them. For example, the empirical formula for both acetylene and benzene is CH, but the molecular formula for acetylene is C2H2, and the one for benzene is C6H6.
Types of Compounds There are three types of compounds: metals, nonmetals, and semimetals. Metals, such as gold, copper, and iron, are ductile (able to be drawn into wires), malleable (can be hammered into thin sheets), have a metalic shine/luster, and are good conductors of electricity. Nonmetals, such as oxygen and sulfur, are brittle (break into peices when hammered), have no shiny luster, and are poor conductors of heat and electricity. Semimetals, such as boron and silicon, have mixed metal and nonmetal characteristics.