SELECTED PRINCIPLES: INTRODUCTION - LOCALIZED COVALENT BONDING (1) Bonding between atoms, which occurs because the resulting molecule or compound has a lower energy than its constituent atoms, is achieved by redistributing the valence (or bonding) electrons. In covalent bonding, this redistribution occurs by the atoms sharing two or more electrons. The term localized covalent bond describes the mutual electrostatic attraction of two adjacent nuclei for a shared pair of electrons which occupy the same molecular energy level.
The Mechanism of Covalent Bonding Consider two hydrogen atoms, Ha and Hb. The (potential) energy of each electron, in its own separate atomic energy level, is reduced when both electrons enter a molecular energy level which encompasses both nuclei; in doing so, and forming a localized covalent bond, potential energy is transduced to heat energy. So, bond formation is an exothermic process: Ha(g) + Hb(g) 覧覧覧覧覧ｮ H覧H(g) DH = -432 kJ mol-ｹ *
Localized Single Bonds The sketch graph above includes a method of representing the electronic structure of dihydrogen which is a fairly reasonable approximation to physical reality; in particular, it emphasizes that - after bonding - the two electrons are indistinguishable from one another and that they occupy one molecular energy level (not separate atomic energy levels). However, similar representations for more complex molecules are rarely used because the resulting diagrams are inevitably more complicated.
Shown below, for dihydrogen, is the simplest type of electron-structure diagram.
The merits of this type of electron-structure diagram are two-fold. First, the retention of the atomic energy levels simplifies 'electronic book-keeping'. And second, there is a clear and direct relationship to one standard method of representing a molecule: i.e., by its structural formula.
* Conversely, bond breaking, as in the dissociation of dihydrogen, is an endothermic process; H覧H(g) 覧覧覧覧覧ｮ Ha(g) + Hb(g) DH = +432 kJ mol-ｹ
Shown below are electron-structure diagrams for atomic fluorine and molecular fluorine (or difluorine).
A fluorine atom contains seven electrons in its outermost atomic energy level; because one of these is unpaired, such an atom is known as a free-radical. The potential energy of this unpaired electron is reduced when it becomes part of a covalent bond; as occurs, for example, when a molecule of difluorine is formed. The covalent bond in difluorine is formed in the same manner as that in dihydrogen. Thus, one electron from the outermost atomic energy level of each fluorine atom enters a molecular energy level which encompasses both nuclei; the other (non-bonding) electrons form lone-pairs. Difluorine is an example of a molecule which conforms to the so-called 'octet rule'; i.e., an atom - other than H, He, Li, Be, or B - tends to form bonds until it is surrounded by eight valence electrons. [This is a useful rule of thumb for the electronic structures of molecules which contain only carbon, nitrogen, oxygen, or fluorine. Nevertheless, there are innumerable examples of compounds which contain bonded atoms that have either an 'incomplete' or an 'expanded' octet.] *
Shown below are electron-structure diagrams for hydrogen fluoride and water.
Because the bonds in these two molecules are formed in the same manner, and for the same reason, as both dihydrogen and difluorine, one might expect no further comment. But, together, these four molecules beg an important question: are all covalent bonds the same? The short answer is no! However, in an introductory text, quite how much detail should accompany this flat answer is difficult to judge; and so, rather than throwing caution to the winds, only three brief points are made here. First, by analogy with the different energy levels observed in atoms, one would expect there to be different molecular energy levels. Indeed, although well beyond the scope of this text, both spectroscopic studies and theoretical calculations provide support for such an expectation; e.g., the molecular energy levels associated with the covalent bonds in H-H, F-F, H-F, and H-O-H are all different. Second, perhaps intuitively, one might expect that the electrons are not equally distributed when the two nuclei are different; the unequal distribution of 'electron density' is known as polarization (and occurs in both H-F and H-O-H, for example: but not in either H-H or F-F). And third, quantitative determinations of molecular energy levels and polarization, amongst other bonding parameters, allow many reactions of molecules to be both correlated and predicted.
* As a caveat to the 'octet rule', consider the following. Each of the 46 chromosomes present in (most) cells of the human body contain bonded phosphorus which does not adopt the electronic structure of argon. [... And setting aside identical sprogs, the exponentially increasing human population is? ... And setting aside the depressing fact that numbers are ever decreasing, because of pollution, the number of species is?]
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