As discussed previously, electrons are most stable when they exist in fully-paired configurations. The noble gases, which are found in Group 18, or 8A, on the periodic table, are the only elements that naturally possess octets and, consequently, exist independently as monatomic (single-atom) "compounds." The elements that exist in all of the other columns on the periodic table cannot naturally achieve an octet configuration, as they do not inherently contain eight valence electrons. As a result, these elements are not stable by themselves, and instead must form bonds with other elements, in order to achieve stable valence electron configurations.
The previous sections of this chapter have all related to ionic bonding, which requires an electrostatic attraction between charged particles, or ions. Recall that, in order to ionize, or become charged, an atom must gain or lose valence electrons, thereby creating an imbalance between the number of positively-charged protons and negatively-charged electrons that it contains.
However, elements that are classified as metalloids, as well as the non-metals that are found in Group 14, or 4A, on the periodic table, are unable to ionize, as discussed in Section 3.5. Instead, these elements achieve stable electron configurations by sharing valence electrons with one or more other atoms. The result of this type of interaction is called a covalent, or molecular, bond. Specifically, covalent bonds can be formed by combining
- two non-metals or
- a non-metal and a metalloid.
Note that all non-metals are able to bond covalently. Therefore, the non-metals in Groups 15 (5A), 16 (6A), and 17 (7A) on the periodic table are able to achieve stable electron configurations through both ionic and covalent interactions.
The remaining sections of this chapter will explain how electrons are shared between atoms and will use two-dimensional pictures, called Lewis structures, to visually-represent the resultant covalent bonds. Additionally, the processes for determining the chemical formulas and the chemical names of the corresponding covalent molecules will be presented and applied.
Finally, it is critically-important to note that because ionic and covalent bonds are formed through completely-unrelated electronic interactions, namely the transfer and sharing of electrons, respectively, the formula-development and naming procedures that will be outlined for covalent molecules will be fundamentally different than the ionic processes that have been previously discussed.