Atoms can join together by forming a chemical bond, which is a strong, persistent attraction between two atoms. Chemical bonds are formed when electrons in different atoms interact with each other to make an arrangement that is more stable than when the atoms are apart.

What causes atoms to make a chemical bond with other atoms, rather than remaining as individual atoms? A clue comes by considering the noble gas elements, the rightmost column of the periodic table. These elements—helium, neon, argon, krypton, xenon, and radon—do not form compounds easily, which suggests that they are especially stable as lone atoms. What else do the noble gas elements have in common? Except for helium, they all have eight valence electrons. Chemists have concluded that atoms are especially stable if they have eight electrons in their outermost shell. This useful rule of thumb is called the octet rule, and it is a key to understanding why compounds form.

There are two ways for an atom that does not have an octet of valence electrons to obtain an octet in its outer shell. One way is to transfer electrons between atoms until all atoms have octets. Because some atoms will lose electrons and some atoms will gain electrons, there is no overall change in the number of electrons, but individual atoms acquire a nonzero electric charge. Since an atom is neutral by definition, a new word ion is used to refer to an atom or small group of atoms that carries a charge.  An atom that loses electron(s) becomes a positively-charged cation.  An atom that gains electrons becomes a negatively-charged anion.  Note that the charges for ions are always due to gain or loss of electrons, never gain or loss of protons which are at the center of the atom and key to the atom’s identity.  Because opposite charges attract, anions and cations attract each other, forming ionic bonds. The resulting compounds are called ionic compounds and are the primary subject of this chapter.

The second way for an atom to obtain an octet of electrons is by sharing electrons with another atom. These shared electrons simultaneously occupy the outermost shell of more than one atom. The bond made by electron sharing is called a covalent bond. Covalent bonding and covalent compounds will be discussed in Chapter 4 “Covalent Bonding and Simple Molecular Compounds”.