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Covalent Bonding
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Metal atoms can donate electrons to nonmetal atoms, but nonmetal atoms do not form monatomic positive ions because they would have to donate too many valence electrons to form octets. (Single nonmetal atoms do not donate electrons at all, but some groups of nonmetal atoms can. This will be discussed later in this section.) Nonmetal atoms can accept electrons from metal atoms if such atoms are present; otherwise, they can attain an octet by electron sharing. A covalent bond consists of shared electrons. One pair of electrons shared between two atoms constitutes a single covalent bond, generally referred to as a single bond. An unshared pair of valence electrons is called a lone pair. Elements or compounds bonded only by covalent bonds form molecules. So, a covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding.Consider the hydrogen molecule, H2. Each atom of hydrogen has one electron and would be more stable with two electrons (the helium confi guration). There is no reason why one hydrogen atom would donate its electron and theother accept it. Instead, the two hydrogen atoms can share their electrons:
Electrons shared between hydrogen atoms are counted toward the duets of both atoms. In the hydrogen molecule, each hydrogen atom has a total of two electrons in its fi rst shell and, thus, a stable confi guration. Electrons shared between other nonmetal atoms are counted toward the octets of both.
In this case, then, the hydrogen atoms will position themselves so that the system will achieve the lowest possible energy; the system will act to minimize the sum of the positive (repulsive) energy terms and the negative (attractive) energy term. The distance where the energy is minimal is called the bond length. In electron dot diagrams for uncombined atoms, the four areas around the symbol can hold a maximum of two electrons each. However, be aware that up to three pairs of electrons can sometimes be placed between covalently bonded atoms. Another representation of molecules is the structural formula, in which each electron pair being shared by two atoms is represented by a line or dash.Electrons not being shared may be shown as dots in such a representation. Structural formulas for H2 and HCl are
Two atoms can share more than one pair of electrons to make an octet for each atom. Consider the nitrogen molecule:
In this case, three electron pairs are shared, and each nitrogen atom has an octet of electrons. There is one lone pair of electrons on each nitrogen atom. Three pairs of electrons shared between the same two atoms constitute a triple bond. If two pairs of electrons are shared, a double bond results. Consider the carbon dioxide molecule:
The term multiple bond refers to either a double or triple bond. With a few simple rules, recognizing compounds that consist of molecules is fairly easy. All compounds that are gases or liquids at room temperature are molecular. (Solid compounds may be molecular.) Most compounds that do not have a metal atom or an ammonium ion (NH4+) in them are molecular. When not combined with other elements, most nonmetallic elements form molecules. (The noble gases have monatomic molecules; their atoms are uncombined. The bonding results from the mutual attraction of the two nuclei for the shared electrons. Between these extremes are intermediate cases in which the atoms are not so different that electrons are completely transferred but are different enough that unequal sharing results, forming what is called a polar covalent bond.
Donor-Acceptor Bond (DAB) – special type of covalent bond DAB (also coordination bond), a term denoting one of the ways in which a chemical covalent bond is formed. The ordinary covalent bond between two atoms is due to the interaction of two electrons, one from each atom. The donor-acceptor bond is formed by a pair of electrons from one atom (the donor) and a free (unfilled) orbital from another (the acceptor). The difference can be expressed schematically as
| Covalent bond | Donor-acceptor bond |
| A + B → A: B | A: + B → A: B |
In both cases the electrons become common to two atoms. A typical example is the formation of an ammonium ion by reaction of ammonia with a proton:
In the ammonia molecule the nitrogen atom has an unshared pair of electrons, and in the hydrogen ion the 1s orbital is free. If the NH3 molecule and the H+ ion approach sufficiently closely, the two-electron cloud of the nitrogen enters the sphere of attraction of the hydrion and becomes common to the nitrogen and hydrogen ions — that is, a fourth covalent N—H bond forms. All the N - H bonds in the ion acquire the same value and become identical. Another important example is the formation of the oxonium ion (hydronium-ion):
In this case the donor is the water molecule, and a proton is the acceptor. This means of formation of covalent bonds plays an important role in the chemistry of complex compounds.