Chemical Bonds

Elements that do not have a noble-gas configuration (a stable configuration) try to attain such a configuration by entering into chemical reactions. Stable molecules are formed when atoms combine so as to have outer shells holding eight electrons.

If atoms are no more than a few electrons away from a stable configuration, they generally attain it by losing or gaining electrons to form electrically charged particles called ions. Positively charged ions (formed by a loss of electrons) are called cations, and negatively charged ions (formed by an electron gain) are called anions. Ions seldom have a charge greater than three, which means that atoms seldom gain or lose more than three electrons.

Table salt is composed of sodium and chlorine ions. The sodium atom loses its one outer electron to become a positively charged sodium ion. Its outer shell now contains eight electrons. The chlorine atom gains one electron in the outer shell, giving a total of eight electrons, to become a negatively charged chloride ion. The positive and negative ions attract each other and form a solid crystal.

The electrons in the outer shell of an element are called valence electrons. Valence electrons are those electrons that are available to form bonds with other atoms. Groups of elements with similar electron configurations (arrangements of electrons in their orbitals) behave in a similar way in chemical reactions, so these groups have similar chemical and physical properties. These groups of elements are called families (see Periodic Law). The periodic table shows how elements can be grouped into families. Elements with atoms that have one valence electron are in Group I; elements with two valence electrons are in Group II; elements with six are in Group VI; and elements with seven are in Group VII.

The periodic table helps chemists to remember the similarities and gradation of properties within element groups. The discovery of the periodic law and publication of this table in 1869 by Russian chemist Dmitri I. Mendeleyev was a major step in organizing information about the known elements and in predicting the properties of unknown ones.


Ionic Bonds
Oppositely charged ions have a strong mutual electrostatic attraction when brought together, but, if brought too close, the electron clouds repel each other. Thus, a pair of mutually attracted ions will maintain a certain distance from each other. This distance is called the bond length, and the electrostatic attraction of the ions constitutes an ionic (or electrovalent) bond. Ionic bonds are very common and are exemplified by table salt, in which a sodium ion attracts a chloride ion to form Na+Cl- or, as usually written, NaCl. Calcium ions (Ca2+) and chloride ions (Cl- ) combine in a one-to-two ratio to form calcium chloride, CaCl2. The total charge on each combination of ions, NaCl and CaCl2, is neutral, or zero.

Covalent Bonds
Another common type of bond, the covalent bond, results when two atoms share one or more pairs of electrons in an attempt to fill their outer shells and become more energetically stable. The atoms are held together by the mutual electrostatic attraction between the protons in their nuclei and these electrons. The bonded atoms form a stable unit called a molecule.

For example, because a chlorine atom is one electron short of completing its outer shell (and attaining a noble-gas configuration), two chlorine atoms combine to form a chlorine molecule by sharing two electrons. The atoms thereby complete each other’s outer shell: Cl Cl Cl2. Electron sharing distinguishes a covalent bond from an ionic bond (in which the electrostatic attraction of the ions results in bonding). In an ionic compound there are no molecules—only charged particles composing an extensive three-dimensional array.

Covalent bonds tend to form when the bonded atoms have nearly the same attraction for electrons; ionic bonds form when the electron-attracting power of the atoms differs markedly.