A chemical bond is the link between atoms. When a chemical bond forms
between two atoms the resulting arrangement of the two nuclei and their
electrons has a lower energy than the total energy of the separate atoms. If
the lowest energy can be achieved by the complete transfer of one or more electrons
from each atom of one element to those of another, then ions form and the
compound is held together by the electrostatic attraction between them. This
attraction is called an ionic bond. Sodium and chlorine atoms, for example, bond
together as ions because solid sodium chloride, which consists of Na and Cl
ions, has a lower energy than a collection of widely separated sodium and chlorine
atoms. If the lowest energy can be achieved by sharing electrons, then the atoms
link through a covalent bond and discrete molecules are formed. Hydrogen and
nitrogen atoms bond together as molecules of ammonia, NH3, for example,
because a gas consisting of NH3 molecules has a lower energy than a gas consisting
of the same number of widely separated nitrogen and hydrogen atoms. A third type
of bond is the metallic bond, in which large numbers of cations are held together
by a sea of electrons. For example, a piece of copper consists of a stack of copper
ions held together by a sea of electrons, each of which comes from one of the
atoms in the sample (recall Fig. 2.27). We consider the metallic bond in more detail
in Chapters 6 and 7. In this chapter we examine ionic and covalent bonds.
The changes in energy responsible for the formation of bonds occur when the
valence electrons of atoms, the electrons in the outermost shells, move to new
locations. Therefore, bond formation depends on the electronic structures of
atoms discussed in Chapters 1 and 2.