Electron pairs in covalent bonding

These representations are all Lewis structures, formulas in which atomic symbols represent nuclei and inner-shell electrons, dot-pairs or dashes between two atomic symbols represent electron pairs in covalent bonds, and dots adjacent to only one atomic symbol represent unshared electrons. It is... 

Gilbert N. Lewis recognized the similarity in behavior of boron tri-fiuoride and a transferred proton toward a base, and in 1923 enunciated a definition of acid-base reaction in terms of sharing of an electron pair—fl base donates an electron pair in covalent bonding and an acid accepts the pair. The acid is called an ELECTROPHILE, and the base is called a nucleophile. In the base, the atom with the unshared pair of electrons is an electron-rich site, and, in the acid, the atom that accepts the pair of electrons to form a covalent bond is an electron-deficient site. The Lewis theory focuses attention on the electron pair rather than on the proton, and in so doing broadens the concept of acidity. The transferred proton of a so-called Brbnsted acid is a special case of a Lewis acid. 

The solution that has been devised for this problem is to imagine that the shared electron pairs in covalent bonds between different elements are completely transferred to one of the two bound atoms. The transfer is assumed to occur to the more electronegative of the two. 

Writing the equation in the usual way directs too much attention to the atoms and not enough to the electrons We can remedy that by deleting any spec tator ions and by showing the unshared electron pairs and covalent bonds that are made and broken Both sodium hydroxide and sodium fluoride are com pletely ionized in water therefore Na" which ap pears on both sides of the equation is a spectator ion Hydrogen fluoride is a weak acid and exists as undissociated HF molecules in water... 

If electron-pair, or covalent, bonding is periodic in two or three dimensions, crystals result. The most important case is the carbon-carbon bond. If it is extended periodically in two-dimensions the result is graphite in three-dimensions it is diamond. Other elements that form electron-pair bonds are Si, Ge, and a-Sn. Some binary compounds are A1P (isoelectronic with Si),... 

The Permanent Dipole Moment. In a pure, single covalent bond between two atoms, the bonding electrons are shared equally between the atoms they belong equally to both nuclei. This equal sharing of the electron pair in the bonding molecular orbital is present in homonuclear molecules such as H2 and 02. However,... 

FC = number of valence electrons of the atom - number of lone pair electrons on this atom - half the total number of electrons participating in covalent bonds with this atom. 

Learning Goal The shape of a molecule plays a large part in determining its properties and reactivity. We may predict the shapes of various molecules by inspecting their Lewis structures for the orientation of their electron pairs. The covalent bond, for instance, in which bonding electrons are localized between the nuclear centers of the atoms, is directional the bond has a specific orientation in space between the bonded atoms. Electrostatic forces in ionic bonds, in contrast, are nondirectional they have no specific orientation in space. The specific orientation of electron pairs in covalent molecules imparts a characteristic shape to the molecules. Consider the following series of molecules whose Lewis structures are shown. 

When two atoms are joined to make a chemical compound, the force of attraction between the two atoms is the chemical bond. Ionic bonding is characterized by an electron transfer process occurring before bond formation, forming an ion pair. In covalent bonding, electrons are shared between atoms in the bonding process. Polar covalent bonding, like covalent bonding, is based on the concept of electron shar-... 

Bonding Pairs and Lone Pairs In covalent bonding, as in ionic bonding, each atom achieves a full outer (valence) level of electrons, but this is accomplished by different means. Each atom in a covalent bond counts the shared electrons as belonging entirely to itself. Thus, the two electrons in the shared electron pair of H2 simultaneously fill the outer level of both H atoms. The shared pair, or bonding pair, is represented by either a pair of dots or a line, H H or H—H. 

Nitrogen is in Group V of the Periodic Table, and so has five outer-shell electrons. Three of these electrons participate in covalent bonding the other two form a non-bonded electron pair, which enables nitrogen compounds (such as ammonia. Figure 3.3 overleaf) to act as nucleophiles in reactions. This is illustrated by Reaction 3.2, between ammonia and iodomethane, giving methylammonium iodide. 

The ten covalent bonds in the Lewis structure shown account for 20 valence electrons, which is the same as that calculated from the molecular formula (CsFIg). The eight hydrogens of CaHg contribute 1 electron each and the three carbons 4 each, for a total of 20 (8 from the hydrogens and 12 from the carbons). Therefore, all the valence electrons are in covalent bonds propane has no unshared pairs. 

The A1 atom is surrounded by three covalent bond electron pairs and two dative bond electron pairs. According to the VSEPR model the most stable arrangement of five electron pairs in the valence shell of the central atom is trigonal bipyramidal, and this is indeed the structure observed. The VSEPR model may also be used to rationalize the observation that the donor atoms occupy axial positions an axial bond electron pair is repelled by three (equatorial) bond pairs, while an equatorial bond electron is repelled by two axial bond pairs across the same angle. Since a covalent bond electron pair requires more space at the aluminum atom than a dative bond electron pair, the covalently bonded atoms occupy the equatorial positions. 

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. 

For a bonded atom, the count of its electrons (engaged in covalent bonds, in free electron pairs and unpaired) may differ from the number of valence electrons of a free atom of the same element. The difference, if ciny, is the atom s charge. 

Stabilization arising from the electron pair in the bonding MO is canceled by the destabilization due to the electron pair in the anti bonding MO. From its zero bond order [j(2 — 2) = 0], we predict, and experiment has so far confirmed, that a covalent He molecule does not exist. 

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