An atom having an incomplete outer shell can satisfy the octet rule in one of three ways:
1. It can gain electrons from another atom.
2. It can lose electrons to another atom.
3. It can share one or more electron pairs with another atom.
Such interactions among atoms result in chemical bonds, forces that hold atoms together. If the force is caused by the attraction of oppositely charged particles formed by the gain or loss of electrons, the bond is called ionic (eye-ON-ick). If the force is caused by the electrical attraction created by atoms sharing electrons, the bond is called covalent (ko-VAY-lent). Other weaker kinds of bonds also occur.
Ions and Ionic Bonds
Electrons stay in their shells because they are attracted to the positive charge of the nucleus. However, electrons far from the nucleus are not held as tightly as electrons closer to the nucleus. In addition, they have more energy and they interact with other atoms more easily than close, tightly held, energy-poor electrons. Therefore, atoms typically interact with other atoms by means of the electrons in their outermost (highest) energy levels, or shells. The types of interactions that occur tend to result in atoms with completed outer shells, thus satisfying the octet rule.
An atom with a nearly completed outer shell tends to "take" enough electrons to complete its outer shell from an atom that has only one or two electrons out of the eight needed for completion of its outer shell. Once these outer electrons are gone from the atom giving up electrons, the next shell in becomes its new, complete outer shell. As a result of this interaction, the number of protons no longer equals the number of electrons in either atom. The atom taking on electrons acquires a negative charge. The atom giving up electrons acquires a positive charge. They are no longer called atoms because they are not electrically neutral anymore. Such charged particles are called ions (EYE-onz).
Figure 3 The formation of an ionic bond.
Figure 3a illustrates the formation of ions with a specific example of electron "give and take." Notice in the upper left portion of the illustration that sodium (Na) is an element with 11 protons and 11 electrons. It is a soft, silver-white metal that occurs in nature as a part of ionic compounds. One familiar compound is sodium chloride, or table salt. Of sodium's 11 electrons, 2 are in its innermost energy level (full with 2 electrons), 8 are at the next level, and 1 is at the outer energy level. Because of this distribution of electrons, its outer energy level is not full, and therefore the octet rule is not satisfied.
Chlorine (Cl) is an element with 17 protons and 17 electrons, as shown in the upper right portion of the illustration. In its molecular form (Cl2), it is a greenish-yellow gas that is poisonous and irritating to the nose and throat. In the ionic compound sodium chloride, however, it does not have these characteristics. Of chlorine's 17 electrons, 2 are at its innermost energy level, 8 at the next energy level, and 7 at the outer energy level. Chlorine, like sodium, has an outer energy level that is not full. Sodium and chlorine atoms can interact with one another in a way that results in both having full outer energy levels.
When placed together, the metal sodium and the gas chlorine react explosively. The single electrons in the outer energy levels of the sodium atoms are lost to the chlorine, as shown in the lower portion of Figure 3a. The result is the production of Na+ and Cl– ions. These ions come together as their opposite charges attract one another. This type of attraction is called electrostatic attraction and results in ionic bonding of the sodium and chloride ions. As these ions are drawn to one another, they form geometrically perfect crystals of salt as shown in Figure 3b.
The transfer of electrons between atoms is an important chemical event-one type of chemical reaction. In fact, this type of chemical interaction has a special vocabulary. When an atom loses (gives up) an elec-tron, it is oxidized. The process by which this occurs is called an oxidation (ok-si-DAY-shun) meaning "to combine with oxygen." The name reflects that in biological systems, oxygen, which strongly attracts electrons, is the most frequent electron acceptor. Therefore, atoms that give up electrons to oxygen are "acted upon" by oxygen, or oxidized. For example, when iron combines with oxygen in the presence of moisture, it becomes oxidized. The product of this oxidation is commonly known as rust.
Conversely, when an atom gains an electron, it becomes reduced. The process is called a reduction. This name reflects that the addition of an electron reduces the charge by one. For example, if a molecule had a charge of +2, the addition of an electron (-1) would reduce the molecule's charge to +1.
Oxidation and reduction always take place together because every electron that is lost by one atom (oxidation) is gained by some other atom (reduction). Together they are called redox reactions. In a redox reaction, the charge of the oxidized atom is increased, and the charge of the reduced atom is lowered.