Ionic and Covalent Bonding
As we mentioned on the previous page, all atoms "want" to have a filled set of s and p subshells, just as the noble gases do. There are two ways to do this: form an ion, or share electrons with another atom.
For the alkali metals, alkaline earth metals, and some of the transition metals, it is easier to lose electrons to achieve a noble-gas configuration than gain enough to fill their s and p subshells. Elements in the alkali metals lose one electron; those in the the alkaline earth metals lose two. The situation in the transition metals group is more complicated, and we'll return to it later.
Atoms on the right side of the Periodic Table must gain electrons to reach a noble gas configuration. Because the s and p subshells (which contain a total of eight electrons) make up most atoms' valence electrons, each atom must make enough bonds to have eight valence electrons. This is called the octet rule. Some atoms, however, do not obey the octet rule: boron, for instance, only makes three bonds, and atoms past phosphorus can make more bonds than necessary to satisfy the octet rule.
The first way for these atoms to gain electrons simply take them from another atom; for example, chlorine wants to gain one electron to attain an argon configuration, and one way it can do this is to steal sodium's electron. In this way, both atoms are happy, and the attraction between the positive ion (the cation) and the negative ion (the anion) will hold the atoms together. Such a bond type is called an ionic bond. The halogen need only one electron; each column to the left needs one more electron to complete its subshells.
An ion will bond to as many other ions as necessary to neutralize its electric charge--for instance, magnesium forms a 2+ ion, so it will bond with two chlorine ions. Oxygen will form a 2- ion, so it needs to bond with only one magnesium 2+ ion. Iron sometimes forms a 3+ ion, and since oxygen has a 2- charge, two iron cations must bond with three oxygen anions, giving a molecule of Fe2O3.
Two atoms that want to gain electrons may also share valence electrons; two fluorines will bond, sharing one electron each, so each atom seems to "think" that it has eight, giving full s and p subshells. This type of electron-sharing bond is called a covalent bond. Atoms may share more than one electron pair: in the O2 molecule, two electron pairs are shared, and three electron pairs are shared between the nitrogen atoms in the N2 molecule. Covalent bonds involving the sharing of more than three pairs of electrons are rare.