Chemical Bonding, Page 2

Covalent Bond

A Covalent Bond is a bond formed by the sharing of one, two or three pairs of electrons by two atoms. The bonded atoms will experience some level of increased stability as a result of the sharing process. The shared electrons will reside primarily between the two bonded nuclei. This will produce an electrostatic force of attraction between the nuclei and the shared electron cloud. This force holds the bonded system together. The more electrons occupying the area between the nuclei, the greater the attraction and the stronger the bond. Consequently, all other factors being equal, a triple bond is stronger than a double bond which is stronger than a single bond. It is not possible to ever have more than three pairs of electrons between the two bonded nuclei. Any number beyond three pairs would increase the area of negative charge so much that the extra electron pairs will always be pushed out of the region and the bond would be destroyed.   One theory of covalent bonded is the Molecular Orbital theory. This theory states that in the covalent bonding process new orbitals, called Molecular Orbitals, are created from the original atomic orbitals that were located on the bonding atoms. In the covalent bond process, the electrons are not always shared equally. If there is a substantial difference between the electronegativities of the two bonded atoms, then the electrons will reside closer to the atom with the higher electronegativity. This will lead to a covalent bond that is polar. When the atoms have similar electronegativity values, then the electrons are shared more equally and the bond is referred to as being nonpolar.

Double Bond

A Double Bond is a covalent bond that consists of two shared electron pairs, or a total of four electrons. This bond has a bond order of two. The double bond is in between the triple bond and single bonds in terms of bond strength. While it is a strong bond, it is also a reactive bond because of the way it is constructed.

Ionic Bond

An Ionic Bond is a bond formed by the transfer of one or more electrons from an atom with a low electronegativity to an atom with a high electronegativity. The process leads to the formation of cations and anions. The mechanism that holds an ionic bond together is the electrostatic force of attraction between the negative anion and the positive cation. Unlike the covalent bond which creates new orbitals, the ionic bond simply rearranges the electrons within the existing atomic orbitals. Also, the ionic bond is not stereospecific, but is omnidirectional. As a result, ionic systems tend to react faster because of less restrictions on the locations of the bonding atoms. Ionic bonding is the extreme case of polarity in that the large difference of electronegativities between the atoms has allowed one atoms to become fully positively charged while the second atom is fully negatively charged.

Ionic Character

Ionic Character refers to the amount of time that a bond exists in the ionic form. Bonds fluctuate between existing in ionic forms and in covalent forms. This is primarily a result of the random nature of electrons. An any given instant a pair of shared electrons may suddenly move to a specific atom and, in essence, become transferred or now exist in the ionic form. The reverse may also happen. The amount of time that a bond exists in the ionic form is measured by the percent of ionic character. The numerical value of the percent of ionic character is related to the difference in electronegativity values for the two bonded atoms. If the difference is greater than 1.7, then the bond will be primarily ionic. If the difference is between 1.7 and 0.3, then the bond will be primarily polar covalent. If the difference is below 0.3, then the bond is said to be nonpolar covalent.  The relationship between the difference in the electronegativity values of two bonded atoms and the percent of ionic character can be expressed graphically. The relationship is fairly straightforward.   Inversely related to ionic character is covalent character. The greater the ionic character of a bond, the less the covalent character of the same bond.

Molecular Orbital

Molecular Orbitals are the new orbitals that are created when an covalent bond forms. The molecular orbitals (m.o.'s) are created by combining the original valence level atomic orbitals that were involved in the bonding process. The new orbitals will concentrate the electron density between the bonded nuclei. Unlike atomic orbitals which are quite predictable in terms of their shape, m.o.'s are not quite as predictable. Their shapes will be unique to the specific systems that they appear in. Examples are

Nonpolar Covalent Bond

A Nonpolar Covalent Bond is formed when two atoms with very similar electronegativities undergo covalent bond formation. If the difference between the electronegativity values for the two bonded atoms is less than 0.3, then the electrons involved in the bond are shared almost equally. As a result, the high degree of symmetry in the structure prevents either end of the structure from experiencing increased negative or positive quality. As a nonpolar system, it will not respond to any areas of charge that might be near the system. Consequently, nonpolar systems have their own unique ways of acting and reacting. This is all made clearly when compared to the Polar Covalent Bond.

Omnidirectional

The term Omnidirectional literally means "in all directions." It refers to chemical phenomena that are independent of location or orientation in three-dimensional space. For instance, ionic bonds are omnidirectional. Because a cation radiates positive charge in all directions, it can attach to an anion coming at in from any direction at all. This is unlike covalent bonds which will only form when the bonding atoms are aligned at very specific locations.

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