Electronegativity negative pole

Problem 11.49 Why are the negative poles of the dipole moments of C H,OCH, and C H,NH, in the ring away from the O and N, even though these atoms are more electronegative than C ... 

When unequal electronegativities of two atoms involved in a bond result in charge separation as just described, we say that the bond is polar. Hydrogen chloride has a polar bond. The charge separation results in a dipole, that is, a positive and a negative pole" in the molecule. The product of the amount of charge separation (e) times the distance of the charge separation (d) is called the dipole moment (p,). 

Polarity is a familiar concept. Batteries have positive poles and negative poles. Magnets have north poles and south poles. When the electrons preferentially sit on one end of a molecule because that is where the electronegative elements are, the molecule has a positive pole and a negative pole thus, it has polarity. A molecule with a more positive end and a more negative end is said to have a dipole as it has two poles. ... 

Polar covalent bonds are called polar because the rmequal electron sharing creates two poles across the bond. Just as a car battery or a flashlight battery has separate positive and negative poles, so polar covalent bonds have poles, as shown in Figure 9.8. The negative pole is centered on the more electronegative atom in the bond. This atom has a share in an extra electron. The positive pole is centered on the less electronegative atom. This atom has lost a share in one of its electrons. Because there was... 

The bond electron pair will by average be located most of the time closest to the chlorine atom because of the larger electronegativity. Thus the chlorine atom in the molecule constitutes a negative pole while the hydrogen atom constitutes the positive pole. Dipole-dipole interactions among the molecules are sketched by the grey lines. 

Note that the negative pole would be on the B atom which is less electronegative than the N atom. The observed dipole moment of 5.22 Debye suggest that the two electrons are not equally shared, they continue to be closer to the N than to the B atom. In the following we shall nevertheless indicate a dative bond by an arrow pointing from the donor to the acceptor atom N -> B. 

The development of valency arose from Berzelius theory of chemical combination which stressed [17, 18] the electronegative and electropositive character of combining atoms. In the mid-nineteenth century, Frankland, Kekule, Couper, Butlerov and Kolbe [19-26], building on the theory of radicals, developed the theory of valency in which elements in compounds were joined by an attraction of positive and negative poles. The concept of valency preceded the discovery of the electron and the planetary view of the atom and may be traced to the 1850 paper by Frankland [19, 24]. He combined the older theories of free radicals and type theory and demonstrated that elements have the tendency to combine with other elements to form compounds containing an integer number of attached elements, e.g. in the three attached atoms M/j, NI3, four attached atoms in CH4 and five attached atoms in PCls. Based on these examples and postulates, Frankland articulated the truism ... 

Since sulfur, carbon, and oxygen are all more electronegative than hydrogen, the electron density in these bonds is shifted away from hydrogen toward the other element. There is essentially no electronegativity difference in the H— P bond, so neither atom will act as a negative pole. 

In contrast, the bent water molecule is polar the two polar bonds do not cancel each other because the molecule is not symmetrical around a horizontal axis (Active Fig. 13.10[b]). The bonding electrons spend more time near the more electronegative oxygen atom, which is the negative pole. The positive pole is midway between the two hydrogen atoms. The molecule is said to be a net dipole. 

In a molecule of HCl, not only are the bonds polar covalent, but because the electrons spend more time with chlorine than hydrogen, the chlorine end of the molecule has a negative charge on it. HCl is a dipole, or polar, molecule because the differences in electronegativity have created the two poles. Referring to the dipole arrow, there is no counterbalance of charges in this molecule and it is classified as polar. 

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