Valence refers to a number assigned to elements that reflects their ability to react with other elements and the type of reactions the element will undergo. The term valence is derived from the Latin word for strength and can reflect an element's strength or affinity for certain types of reactions.
The electrons in an atom are located at different energy levels. The electrons in the highest energy level are called valence electrons. In accord with the octet rule— and to become more energetically stable—atoms gain, lose, or share valence electrons in an effort to obtain a noble gas configuration in their outer shell. The configuration of electrons in an atom's outer shell determines its ability and affinity to enter into chemical reactions.
The valence number of an element can be determined by using a few simple rules relating to an ele ment's location on the periodic table. In ionic compounds (formed between charged atoms or groups of atoms called ions) the valence of an atom is the number of electrons that atom will gain or lose to obtain a full outer shell. In group one of the periodic table, elements are assigned a valence number of 1. A valence number of 1 means that an element will generally react to lose one electron to obtain a full outer shell. Group two elements are assigned a valence number of 2. A valence number of 2 means that a group two element will generally react to lose two electrons to obtain a full outer shell. Group 17 elements are assigned a valence number of negative one (-1). A valence number of -1 means that a group two element will generally react to gain one electron to obtain a noble gas electron configuration. Reflecting an inability to react with other elements, Nobel gases, already maintaining a stable arrangement of electrons, are assigned a valence of zero (O).
The term valence can also refer to the charge or oxidation number on an atom. In magnesium atoms (Mg+2) the valence is +2. An atom or ion with a charge of +2 is said to be divalent.
In covalent compounds the valence of an atom may be less obvious. In this case it is the number of bonds formed, that is, whether the bonds are single, double, or triple bonds. A carbon atom with two single bonds and one double bond carries a valence of four (4). In water (H2O), the valence of oxygen is 2 and the valence of hydrogen is 1. In both cases the valence number gives an indication of the number of bonds each atom forms.
Valence bond theory is similar to molecular orbital theory in that it is concerned with the formation of covalent bonds. Valence bond theory describes bonds in term of interactions between outer orbitals and hybridized orbitals to explain the formation of compounds.
Valence Shell Electron Pair Repulsion (VSEPR) theory is one of the favored models to explain covalent bonds. This theory states that molecules will be shaped so as to minimize the repulsion that takes place between valence electrons. Because they are all negatively charged, valence shell electrons repel one another. VSEPR theory states that the atoms of a molecule will arrange themselves and assume a shape around a central atom so as to minimize repulsion between valence electrons.