Atoms form molecules and compounds by sharing electrons to create chemical bonds. Understanding the nature of this bonding begins by knowing the number of electrons associated with each atom. With the information from a periodic table of the elements, and some straightforward arithmetic, you can calculate the number of electrons based on the chemical formula of a material.
Analyze the chemical formula and write down the element types that comprise the compound as well as the number of atoms of each type. The first example, KNO3, contains the elements potassium (K—1 atom), nitrogen (N—1 atom) and oxygen (O—3 atoms). The second example, SO42-, contains the elements sulfur (S—1 atom) and oxygen (O—4 atoms).
Navigate to the periodic table of the chemical elements (see Resources) and find out the integer atomic number for each element identified in Step 1—the number that appears just above the chemical symbol for each element. In our example, the atomic numbers of the elements potassium (K), nitrogen (N), oxygen (O) and sulfur (S) are 19, 7, 8 and 16, respectively.
Multiply the element’s atomic number by the number of atoms of this type (see Step 1) in the molecule. Repeat for all elements in the molecule, then add up all the products to calculate the number of electrons. In the first example, the number of electrons in KNO3 equals (19 x 1) + (7 x 1) + (8 x 3) = 50. In the second example, the number of electrons in SO42- equals (16 x 1) + (8 x 4) = 48.
Subtract the charge value from the number of electrons obtained in Step 3 if the ion has a positive charge. Add the charge value to the number of electrons (Step 3) if the ion has a negative charge. Skip this step if the molecule has a neutral charge. In our examples, only SO42- is a charged ion; it has the negative charge 2. Add this value to the total from Step 3 to determine the total number of electrons in the molecule: 48 +2 = 50.