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Introduction of Charge Distribution in Molecules

If the electron pairs in covalent bonds were shared and donated absolutely evenly there would be no fixed local charges within a molecule. Though this is correct for the diatomic elements like H2, N2 and O2, most of the covalent compounds display some degree of local charge separation, resulting in the bond and / or molecular dipoles. When centers of positive and negative charge distribution do not coincide a dipole exists.

Formal Charges

A large local charge separation generally results when a shared electron pair is donated unilaterally. The 3 formulas of Kekulé shown here illustrate this condition.

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In the ozone's formula the central oxygen atom has three bonds and a full positive charge whereas the right hand oxygen has only a single bond and is negatively charged. So the overall charge of ozone molecule is 0. Likewise, the formula of nitromethane has positive-charged nitrogen and a negative-charged oxygen, so the total molecular charge again being 0. Finally, the formula of azide anion has two negative-charged nitrogens and one positive-charged nitrogen, so the total charge is being minus one.
Generally, for covalently bonded atoms having valence shell electron octets, if number of covalent bonds to an atom is less than its normal valence it will carry a negative charge. If an atom's number of covalent bonds is greater than its normal valence it will carry a positive charge. Formal charge on an atom may also be computed by the following formula:

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Polar Covalent Bonds

Due to their differing nuclear charges and result of shielding by inner electron shells, dissimilar atoms of the periodic table have distinct affinities for nearby electrons. Electronegativity is the element's ability to attract or hold onto electrons. Electronegativity value's Rough quantitative scale was established by Linus Pauling and some of these are given in the table. A larger number on this scale indicates a greater affinity for electrons. Fluorine has greatest electronegativity of all the elements, and heavier alkali metals like potassium, rubidium and cesium have the lowest electronegativities. It should be noted that the carbon is about in the middle of the electronegativity range, and is little more electronegative than hydrogen.

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While the two distinct atoms are bonded covalently, shared electrons are attracted to the more electronegative atom of bond, resulting in a shift of electron density towards the more electronegative atom. This kind of covalent bond is polar and will have a dipole. The magnitude of the bond dipole and degree of the polarity will be relative to the difference in electronegativity of the bonded atoms. So O-H bond is more polar than a C-H bond, with the hydrogen atom of former being more positive than the hydrogen bonded to carbon. Likewise, C-Cl and C-Li bonds are polar but carbon end is negative in the latter and positive in the former. Dipolar nature of these bonds is frequently indicated by a partial charge notation (δ+/-) or by an arrow indicating to the negative end of the bond.

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Though there is a little electronegativity difference between carbon and hydrogen, the C-H bond is considered as weakly polar at best, and hydrocarbons generally are considered to be non-polar compounds.

The shift of electron density in a covalent bond toward the more electronegative atom or group can be notice in several ways. For bonds to hydrogen, acidity is one criterion. The proton will be more easily transferred to a base (it will be more acidic), if bonding electron pair moves away from the hydrogen nucleus. An evaluation of acidities of the methane, water and hydrofluoric acid is instructive. The Methane is essentially non-acidic, since C-H bond is nearly non-polar. Described above that the O-H bond of water is polar and it is at least 25 powers of ten more acidic than methane. H-F is over the 12 powers of ten more acidic than water as a consequence of the greater electronegativity variations in its atoms.

Electronegativity variations may be transmitted via connecting covalent bonds by an inductive effect. Replacing one of hydrogens of water by a more electronegative atom that increases the acidity of the remaining O-H bond. So, the hypochlorous acid, Cl-O-H is one hundred million times more acidic and hydrogen peroxide, HO-O-H, is the ten thousand times more acidic than water. This inductive transfer of the polarity tapers off as the number of transmitting bonds increases and presence of more than one highly electronegative atom has a cumulative effect. For instance, trifluoro ethanol, CF3CH2-O-H is about 10,000 times more acidic than the ethanol, CH3CH2-O-H.

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