The Covalent Bond Sharing Electrons

Covalent bonds form between atoms with similar electronegativities. In compounds held together by covalent bonds, electrons don t migrate from one atom to another as they do in ionic bonds. Instead, they are shared by the atoms in the molecule. One way to visualize this was proposed by Gilbert Lewis, a chemist at the University of California, Berkeley. His representations of molecular bonds are called Lewis dot structures. These 

Although most chemistry students know about Lewis dot structures, few of them know much about the man who invented those dotty structures. Fewer still can identify his most important contribution to valence bond theory. 

Lewis was an excellent student, earning his Ph.D. at Harvard by the age of 24. He taught for a while at M.I.T., then moved west to Berkeley in 1912 to become the chairman of the chemistry department at the University of California, a position he held until he died. 

Lewis s research was wide-ranging and outstanding. He published important papers on chemical bonds, acid-base theory, and thermodynamics. He also developed firm ideas about howto build a chemistry department. His philosophy of education was something less than egalitarian. Writing in the Journal of Chemical Education, the distinguished chemist Gerald Branch spelled out Lewis s ideas [F]or a chemist to be useful to 

Lewis himself never won a Nobel, although many of his colleagues thought he deserved one for his many contributions to valence bond theory. His key insight came in 1916, fewer than twenty years after J J. Thomson discovered the electron. Linus Pauling—the guru of the chemical bond—summed up Lewis s most important contribution. It was, Pauling said, the idea that the chemical bond consists of a pair of electrons held jointly by two atoms.  

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Delocalization (Section 1 9) Association of an electron with more than one atom The simplest example is the shared electron pair (covalent) bond Delocalization is important in conjugated tt electron systems where an electron may be associated with several carbon atoms... 

Atoms tend to acquire a noble gas configuration either by forming ions or by sharing electrons in covalent bonds. The tendency of atoms to acquire eight valence electrons is known as the octet rule. 

However, many covalent bonds do not equally share electrons such covalent bonds, as pointed out above, are referred to as polar covalent bonds or bonds of partial ionic character. Electronegativity is rated on a relative scale ranging from 4 (most electronegative, fluorine) to 0.7 (least electronegative, cesium) (Table 1.7). In general, the greater the difference in electronegativity between two elements, the more ionic will be the bond between them (Fig. 1.2). 

To make an octet, each oxygen atom needs two more electrons to be added to its original six. To add two electrons, each oxygen atom must share two electrons with the other atom so that the two atoms share four electrons. The covalent bond formed by the sharing of two pairs of electrons is a double bond, shown in the Lewis structures below. 

The atoms in these other compounds share electrons. The chemical bond that results from the sharing of valence electrons is a covalent bond. In a covalent bond, the shared electrons are considered to be part of the complete outer energy level of both atoms involved. Covalent bonding generally occurs when elements are relatively close to each other on the periodic table. The majority of covalent bonds form between nonmetallic elements. 

In other situations, the neutral oxygen atom may share electrons with one or more other atoms, in order to act as though it has a complete valence shell part of the time. These shared electrons represent covalent bonds and result in the formation of molecular compounds, as shown here ... 

The Covalent Bond Atoms of many elements become more stable by sharing electrons. The chemical bond that forms between nonmetal atoms when they share electrons is called a covalent (koh VAY luhnt) bond. Shared electrons are attracted to the nuclei of both atoms. They move back and forth between the outer energy levels of each atom in the covalent bond. So, each atom has a stable outer energy level some of the time. Covalently bonded compounds are called molecular compounds. 

Barker [18] additionally comments, that teachers put far too much emphasis on the Octet rule in order to determine formulas and bindings of chemical species. As a result, the students rely on this rule to deduce formulae. During the lesson unit on ionic bonding, teachers often use this rule, in order to show that some atoms fill their shells through electron transfer instead of sharing electrons in covalent bonding. She further points out, that students are not capable of understanding how ion lattices are formed solely based on this explanation [18]. 

Atoms can form different types of covalent bonds. In a single bond, two atoms are held together by one electron pair. Many compounds are held together by multiple bonds, that is, bonds formed when two atoms share two or more pairs of electrons. If two atoms share two pairs of electrons, the covalent bond is called a double bond. Double bonds are found in molecules of carbon dioxide (CO2) and ethylene (C2H4) ... 

The reacting atoms are represented with numbers of valence electrons equal to their group numbers. In the fluorine molecule, each atom is surrounded by a completed octet. The electron dot picture of the molecule, or Lewis formula, can be simplified by representing the bonding pair of electrons by a line between atoms, and the other pairs as dots surrounding the atoms f F—Ft. The pairs of electrons not shared in the covalent bond are called nonbonded electrons or lone pairs. 

Covalent bonding tends to occur between non-metallic elements and is often described using the metaphor of sharing electrons. A covalent bond is understood to occur when the valence shells of two atoms overlap so that one (or more) pair(s) of electrons falls within the valence shells of both atoms. At more advanced levels this is described in terms of the interaction of atomic orbitals on different... 

We have mentioned all along that in addition to all the beautiful molecules we constructed up to now using shared electron pairs (covalent bonds), there are materials wherein the electron pair in the bond is completely owned by one of the bonding partners, and as such the bond involves two oppositely charged ions. This ionic constitution of the bond can be detected in a variety of ways. For example, if you connect a crystal of common salt, NaCl, to a buzzer and wet the crystal a bit, the buzzer will whistle due to the movement of the ions and the creation of an electric current. A crystal of sugar that is made exclusively of covalent bonds will not whistle even if you drown the sugar in water. Similarly, a solution of an ionic material like NaCl will conduct electricity, while a solution of a covalent material like acetone or sugar will not. 

Most of the time, I use a slight modification of the eiectron-dot formula called the Lewis structural formula it s basically the same as the electron-dot formula, but the shared pair of electrons (the covalent bond) is represented by a dash. The Lewis structural formula is shown in Figure 7-l(c). (Check out the section, Structural formula Add the bonding pattern, for more about writing structural formulas of covalent compounds.)... 

Ionic compounds form when a metal transfers electrons to a nonmetal, and the resulting positive and negative ions attract each other to form a three-dimensional array. In many cases, metal atoms lose and nonmetal atoms gain enough electrons to attain the same number of electrons as in atoms of the nearest noble gas. Covalent compounds form when elements, usually nonmetals, share electrons. Each covalent bond is an electron pair mutually attracted by two atomic nuclei. Monatomic ions are derived from single atoms. Polyatomic ions consist of two or more covalently bonded atoms that have a net positive or negative charge due to a deficit or excess of electrons. 

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