Atoms contain protons, electrons and neutrons. Protons have a positive charge, while electrons have a negative charge. Because all atoms have a neutral charge, the number of electrons in any given atom equals the number of protons. The latter stems from a distinct chemical element's characteristic known as an atomic number. However, molecules called ions can also carry a negative or positive charge—for instance, CO3(-2) or NH4 (+). The existence of ions indicates that during a chemical reaction the substance either loses or gains electrons. As an example, calculate the number of electrons in the molecule KNO3 and the negatively charged ion SO4(2-).
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, SO4(2-), 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 SO4(2-) 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 SO4(2-) 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.