Pam Bogacki, age 13, of Kirkwood, Missouri, for her question:
What does an atom's valence number mean?
Valence was coined from an older word for valiant or strong. In the 19th century, chemists were trying to figure how and why certain atoms are able to combine with others to form molecules. They called this chemical quality "valence." Their early tests suggested that chemical compounds obey simple mathematical laws. Modern chemists agree. However, they now know that, in some compounds, both the math and the valence are very complex.
A molecule is a package of atoms linked together by chemical bonds. The bonds that unite them are electrons. They may be shared or transferred from one atom to another. Valence numbers tell how many electrons are involved and whether they are shared or transferred. The electrons that form these chemical bonds are in the outer shells of the atoms. Hence, their possible valence patterns depend upon the structure of the various atoms.
The normal atom is electrically neutral because the number of positively charged protons in its nucleus equals its negative electrons. In this state, its valence is zero. The electrons orbit the nucleus at an incredible rate. However, this frantic traffic is organized in spherical shells. The first shell is complete with two electrons, the second with eight and larger shells hold more.
Though the normal atom is electrically balanced, it is more stable when its outer shell is complete. Some need only one or two extra electrons, others need as many as seven. The hydrogen atom has only one electron and seeks an extra one to fill its first shell. Oxygen has a second shell of six and seeks two more. These mutual problems are solved when two hydrogens and one oxygen share two electrons which form the pair of chemical bonds in a molecule of water.
This mutual sharing of electrons is called covalant bonding. In forming a water molecule, oxygen has a valence number of two because it shares two electrons. Each hydrogen atom shares one electron and a valence of one. Some people give the entire water molecule a valence of two because it is united by two shared electrons.
When molecules are formed by transferring electrons, they are called ionic bonds. This is because the atoms that gain or lose electrons become electrically unbalanced ions. For example, sodium has one electron in a third shell and chlorine has seven and together they form an ionic bond called sodium chloride, or table salt.
When the sodium atom transfers its lone outsider, it loses a negative charge and becomes a positive ion. The chlorine that gains it becomes a negative ion. In this ionic bond, sodium has a valence number of plus one, chloride has a valence number of minus one.
To a chemist, these valence numbers denoted the numbers of electrons involved and the covalant or ionic structure of this or that molecule. They do not refer to whole atoms or molecules, which all have a valence of zero. Valence can be quite complex and the term may be used to mean different qualities. For this reason, many chemists feel that the term oxidation explains bonding processes more precisely. However, this is the answer to a somewhat different question.