Fluoride hydrogen bonding covalently bonded

The molecular geometry of methane and of methyl fluoride is tetrahedral. In the case of methane, this symmetrical arrangement of polar covalent carbon-hydrogen bonds leads to a canceling of the bond polarities resulting in a nonpolar molecule. As a nonpolar molecule, the strongest intermolecular force in methane is a London force. In methyl fluoride, a fluorine atom replaces one of the hydrogen... 

Although the gas-phase hydrogen-bonded dimer (MeF)2H" is held by a strong hydrogen bond (McMahon and Kebarle, 1986) this is a rare exception to the previous statement regarding covalently bonded fluoride. More typical are the perfluorocarbons, which are among the weakest hydrogenbonding substances known, as their physical properties and uses clearly demonstrate. 

Eormula HE MW 20.006. A very stable polar covalent diatomic molecule H—E bond energy 136.1 kcaPmol at lower temperatures molecules are associated by hydrogen bonding H—bond length 0.92A partial ionic character 40% dipole moment 6.10 D hydrofluoric acid is an aqueous solution of hydrogen fluoride gas. 

Covalent fluorides are often more volatile than the corresponding covalent chlorides because the former have fewer electrons, so the London forces between them are weaker. However, a striking exception is the low volatility of FFF relative to that of HC1 the difference is due to fluorine s ability to take part in hydrogen bonding, which allows the HF molecules to stick together more strongly in the liquid. 

All the xenon fluorides are formed in exothermic reactions. They are reasonably stable, with Xe—F bond energies of about 125 kJ/mol of bonds. For comparison, strong covalent bond energies range from about 170 to 500 kJ/mol, whereas bond energies of hydrogen bonds are typically less than 40 kj/mol. 

PROBLEM 1.5 Hydrogen is bonded to fluorine in hydrogen fluoride by a covalent bond. Write a Lewis formula for hydrogen fluoride. 

In Section 9.3 we learned that hydrogen fluoride is a covalent molecule with a polar bond. There is a shift of electron density from H to F because the F atom is more electronegative than the H atom. The shift of electron density is symbolized by placing a crossed arrow (H—above the Lewis structure to indicate the direction of the shift. For example,... 

FIGURE 11.6 Hydrogen bonding in water, ammonia, and hydrogen fluoride. Solid lines represent covalent bonds, and dotted lines represent hydrogen bonds. 

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