Polarity info. - Polar Covalent Bonds and Dipole Moments....

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Polar Covalent Bonds and Dipole Moments. Ionic and covalent bonding represent the two extreme methods for the formation of chemical bonds between atoms, with the coordinate covalent bonds representing a type of bonding which may be classified, in many cases, as belonging to both classes. When a covalent bond is formed between two atoms of the same element, the electronegativities of both atoms involved in the bond are the same, so that each will exert the same attraction for the shared electron pair. In other words, the electron pair will be shared (in the formal sense, at least) absolutely equally between the two atoms, each atom will control 50% of the total electron density of the electron pair, and the region of greatest electron density will be the mid-point of the bond. Such covalent bonds are termed non-polar bonds. On the other hand, if the two elements forming the covalent bond are not identical (as is the case for the O-H bonds of H 2 O, for example), the more electronegative atom will exert a stronger attraction for the pair of electrons in the bond than will the less electronegative atom. What this means is that the pair of electrons will no longer be equally shared between the two atoms, but will spend more time close to the more electronegative atom; the region of greatest electron density will be displaced from the mid-point of the bond towards the more electronegative atom. Such bonds are called polar bonds. In a polar bond, the effective share of the electron pair belonging to more electronegative atom is effectively more than 50%, while the share of the less electronegative atom is effectively less than 50%. This situation is illustrated in Figure 1.12. When the average position of the electron pair is mid-way between the two nuclei, the charge distribution of the system is symmetrical about its mid-point, so there is no development of partial charge at either end of the bond. However, when the average position of the electron pair is displaced from the mid-point of the bond, the charge distribution of the bond is no longer symmetrical about its mid-point: one end carries a slight excess of negative charge and the other end carries a slight excess of positive charge. Partial charges in molecules are usually indicated by the symbol δ , which is to be read as meaning "some", or "partial", or "a little." Thus, the negative charge acquired by the more electronegative atom in a polar covalent bond is usually designated as
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Polarity info. - Polar Covalent Bonds and Dipole Moments....

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