Coefficients and subscripts are essential components when writing longhand chemical formula compounds or equations. A coefficient, reflecting the number of molecules in a given substance, is a number placed in front of a given molecule’s abbreviation. A subscript, however, reflecting each element’s atomic contribution to a given molecule, appears following or between elemental abbreviations and is typically smaller in size and set below the type line.
The chemical equation for creation of water molecules, or H2O, is one that uses coefficients. In this equation, two molecules of hydrogen, or 2H2, bond with two molecules of oxygen, or 2O2, to yield two molecules of water, or 2 H2O. As this example illustrates, the use of coefficients allows for an accounting of the number of each molecule included in a chemical reaction as well as a means of balancing chemical equations and determining the limiting reagents in a given equation. For example, this reaction, written completely as 2H2 + 2O2 = 2H2O, shows that hydrogen and oxygen must be present in equal proportions to maximize the amount of water molecules produced.
The formula for baking soda, or NaHCO3, gives an example of a subscript. As this formula reflects, there is one atom each of the elements of sodium, or Na, hydrogen, or H, and carbon, or C. The subscript of 3 following the symbol for oxygen, or O, reveals that three atoms of oxygen are needed for every Na, H and C atom to make a complete molecule of baking soda.