Under the temperature and pressure conditions found on Earth, most elements do not occur in the form of individual atoms; instead, their atoms bond with other atoms. (A group of elements known as noble gases are an exception.) Some atoms bond to form ionic compounds, some form molecules, and still others form metallic substances. Why does this happen? Experiments show that electrical forces hold atoms together and bond them to each other. These electrical attractions lower the total energy of the bonded atoms, and this, in turn, generally makes them more stable. Consequently, atoms that are bonded in compounds tend to be more stable than atoms that are free (not bonded).

The Octet Rule and Chemical Bonds

As noted earlier, valence (outer-shell) electrons are generally involved in chemical bonding. The figure below ▼ shows a shorthand way of representing the number of valence electrons for some selected elements. Notice that the elements in Group I have one valence electron each, those in Group II have two valence electrons each, and so on, up to eight valence electron in Group VIII.

The noble gases have very stable electron arrangements with eight valence electrons (except helium, which has two) and, therefore, tend to lack chemical reactivity. Many other atoms gain, lose, or share electrons during chemical reactions, ending up with electron arrangements of the noble gases. This observation led to a chemical guideline known as the octet rule: Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons. Although there are exceptions to the octet rule, it is a useful guide for understanding chemical bonding.

When an atom’s outer shell does not contain eight electrons, it is likely to chemically bond to other atoms to achieve an octet in its outer shell. A chemical bond is a transfer or sharing of electrons that allows each atom to attain a full valence shell of electrons. Some atoms do this by transferring all their valence electrons to other atoms so that an inner shell becomes the full valence shell.

There are three primary types of bonds. When the valence electrons are transferred between the elements to form ions, the bond is an ionic bond. When the electrons are shared between the atoms, the bond is a covalent bond. When the valence electrons are shared among all the atoms in a substance, the bonding is metallic.

Ionic Bonds: Electrons Transferred

Perhaps the easiest type of bond to visualize is the ionic bond, in which one atom gives up one or more valence electrons to another atom to form ions—positively and negatively charged atoms. The atom that loses electrons becomes a positive ion, and the atom that gains electrons becomes a negative ion. Oppositely charged ions are strongly attracted to one another and join to form ionic compounds.

Consider the ionic bonding that occurs between sodium (Na) and chlorine (Cl) to produce the solid ionic compound sodium chloride—the mineral halite (common table salt). Notice in the figure below ▼ that a sodium atom gives up its single valence electron to chlorine and, as a result, becomes a positively charged sodium ion (Na+). Chlorine, on the other hand, gains one electron and becomes a negatively charged chloride ion (Cl-). We know that ions having unlike charges attract. Thus, an ionic bond is an attraction of oppositely charged ions to one another that produces an electrically neutral ionic compound.

The figure above ▲ illustrates the arrangement of sodium and chlorine ions in ordinary table salt. Notice that salt consists of alternating sodium and chlorine ions, positioned so that each positive ion is attracted to and surrounded on all sides by negative ions and vice versa. This arrangement maximizes the attraction between ions with opposite charges while minimizing the repulsion between ions with identical charges. Thus, ionic compounds consist of an orderly arrangement of oppositely charged ions assembled in a definite ratio that provides overall electrical neutrality.

The properties of a chemical compound are dramatically different from the properties of the various elements comprising it. For example, sodium is a soft silvery metal that is extremely reactive and poisonous. If you were to consume even a small amount of elemental sodium, you would need immediate medical attention. Chlorine, a green poisonous gas, is a toxic, lung-irritating gas used as a chemical weapon during World War I. Together, however, these elements produce sodium chloride, the edible flavor enhancer that we call table salt. Thus, when elements combine to form compounds, their properties change significantly.

Covalent Bonds: Electron Sharing

Sometimes the forces that hold atoms together cannot be understood on the basis of the attraction of oppositely charged ions. One example is the hydrogen molecule (H2), in which the two hydrogen atoms are held together tightly and no ions are present. The strong attractive force that holds two hydrogen atoms together results from a covalent bond, a chemical bond formed by the sharing of one or more valance electrons between a pair of atoms. (Hydrogen is one of the exceptions to the octet rule: Its single shell is full with just two electrons.)

Imagine two hydrogen atoms (each with one proton and one electron) approaching one another, as shown in ▼. Once they meet, the electron configuration changes so that both electrons primarily occupy the space between the atoms. In other words, the two electrons are shared by both hydrogen atoms and are attracted simultaneously by the positive charge of the proton in the nucleus of each atom. In this situation the hydrogen atoms do not form ions; instead, the force that holds these atoms together arises from the attraction of oppositely charged particles—positively charged protons in the nuclei and negatively charged electrons that surround these nuclei.

Metallic Bonds: Electrons Free to Move

A few minerals, such as native gold, silver, and copper, are made entirely of metal atoms packed tightly together in an orderly way. The bonding that holds these atoms together results from each atom contributing its valence electrons to a common pool of electrons, which freely move throughout the entire metallic structure. The contribution of one or more valence electrons leaves an array of positive ions immersed in a “sea” of valence electrons, as shown in ▼.

The attraction between this sea of negatively charged electrons and the positive ions produces the metallic bonds that give metals their unique properties. Metals are good conductors of electricity because the valence electrons are free to move from one atom to another. Metals are also malleable, which means they can be hammered into thin sheets, and ductile, which means they can be drawn into thin wires. By contrast, ionic and covalent solids tend to be brittle and fracture when stress is applied. Consider the difference between dropping a metal frying pan and a ceramic plate onto a concrete floor.

• When atoms are attracted to other atoms, they can form chemical bonds, which generally involve the transfer or sharing of valence electrons. The most stable arrangement for most atoms is to have eight electrons in the outermost principal shell. This concept is called the octet rule.

• To form ionic bonds, atoms of one element give up one or more valence electrons to atoms of another element, resulting in positively and negatively charged atoms called ions. The ionic bond results from the attraction between oppositely charged ions.

• Covalent bonds form when adjacent atoms share valence electrons.

• In metallic bonds, the sharing is more extensive: The shared valence electrons can move freely through the substance.

• chemical bond: A strong attractive force between atoms that involves the transfer or sharing of electrons that allows each atom to attain a full valence shell.

• covalent bond: A chemical bond formed by the sharing of one or more valence electrons.

• ionic bond: A chemical bond between two oppositely charged ions formed by the transfer of valence electrons from one atom to the other.

• ions: Atoms or molecules that possess a positive or negative electrical charge.

• metallic bonds: Chemical bonds present in all metals characterized by a type of electron sharing in which the electrons move freely from atom to atom.

• octet rule: A chemical guideline which says that atoms combine such that their outer energy level (shell) contains eight electrons.