Covalent Bonds - Covalent bonds are bonds that are created when atoms share one or more valence electrons. These bonds are stronger than ionic bonds. The number of dashes present in a structural formula represents the number of pairs of shared electrons.
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| Covalent Bonds |
Keep in mind that an ionic bond involves a complete transfer of electrons between atoms. This characteristic is in contrast to another type of chemical bond, one that is probably the most common in living systems. (Ionic bonds, on the other hand, are more characteristic of inorganic compounds.) A covalent bond is formed by the cooperative sharing of valence electrons between two or more atoms, in order to satisfy the orbital configuration of the atoms involved. Atoms held together by covalent bonds form molecules. Because the electrons are attracted to the nuclei of all the atoms involved, a covalent bond is much stronger than an ionic bond. For instance, hydrogen, with its single electron in the first energy level, rarely exists as just a single atom. Instead, hydrogen atoms usually form covalent bonds and exist in pairs, as H2. The two electrons travel around both nuclei, so that in effect each atom has a first energy level with two electrons in it its outermost energy level is full.
Another example of a covalent bond is the methane molecule CH4. Each hydrogen atom has one electron in its outer shell, and carbon has four electrons in its outer shell. By sharing electrons, each hydrogen atom has two electrons in its outer shell (its first energy level), and the carbon has eight in its outer shell (its second energy level).
When chemists refer to the hydrogen molecule, they often write out the notation in what is called a structural formula, as HH. The dash indicates that the two hydrogen atoms share a pair of electrons with each other. The notation HH means the same as H2, but, as you can see, a structural formula provides a more specific representation of the atom's position within the molecule.
Atoms sometimes share more than one pair of electrons. When two atoms share two pairs of electrons, a double bond is formed. When they share three pairs, a triple bond is formed. For example, oxygen has six electrons in its outer shell; it needs two to make a complete shell of eight. If two oxygen atoms share two pairs of electrons that is, form a double bond each will complete its outer shell. The structural formula for this molecule is 0=0 (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology).
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