Polar covalent bonds Hydrogen bonding

Chemists refer to the bond in a molecule like sodium chloride as ionic , meaning that its electron pair resides entirely on chlorine. At the other extreme is the covalent bond in the hydrogen molecule, where the electron pair is shared equally between the two hydrogens. Intermediate cases, such as the bond in hydrogen fluoride which is clearly polarized toward fluorine, are generally referred to as polar covalent bonds (rather than partially ionic bonds). Are these situations really all different or do they instead represent different degrees of the same thing ... 

One important result of polar covalent bonding in some molecules is to encourage hydrogen bonds to form between... 

Water, however, is a wonderful solvent for ionic-bonded substances such as salt. The secret to its success lies in the electric dipoles created by the polar covalent bonds between the hydrogen and oxygen atoms. In water, the polar bonds are asymmetric. The hydrogen side is positive the oxygen side is negative. One measure of the amount of charge separation in a molecule is its dielectric constant. Water has a dielectric constant that is considerably higher than that of any other common liquid. 

The nature of the bonds between an oxygen atom and two atoms of hydrogen has an enormous impact on how our planet works. Because of the highly polar covalent bond, salt dissolves in water, which enabled our ancestors to preserve meat. It also produces the hydrogen bonds that make our lakes freeze from the top down, per-... 

Hydrogen bond A weak bond between the hydrogen in a polar covalent bond and a neighboring molecule with a highly electronegative atom. 

When hydrogen and fluorine combine, a polar covalent bond is formed. Which of these formulas is the best way to express this relationship ... 

FIG. 3 The vapor phase water dimer structure. Polar covalent bonds are shown as solid lines and the hydrogen bond as a dashed line (adapted from Ludwig, 2001). 

Some ionic compounds contain a combination of bonds. For instance, in polyatomic ions such as ammonium (NH4+), the hydrogen atoms are bonded to the nitrogen atom by polar covalent bonds. The ionic bond is thus between this covalently bonded moiety and another oppositely charged ion such as chloride (CT). 

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... 

Other examples of polar covalent bonds include C—O, O—H, and N—H. Carbon and hydrogen attract electrons to almost the same degree. Therefore, when carbon is bonded to another carbon atom or to a hydrogen atom, the bond is not usually considered to be polar. For example, C—C bonds are considered to be non-polar. 

Both reactions involve dissociation of a polar covalent bond to hydrogen and both lead to a "free" proton. While absolute acidities and basicities are rarely if ever (directly) measured experimentally, they provide a good opportunity to assess the performance of different models with regard to the energetics of heterolytic bond dissociation. 

This review has attempted to illustrate the relevance and the widespread utility of the CM model. Indeed, the author believes it is difficult to specify any area of structural or mechanistic chemistry where the CM approach is not applicable. The reason is not hard to find the CM model has its roots in the Schrodinger equation and as such its relevance to chemistry cannot be easily overstated. Even the fundamental chemical concept of a covalent bond derives from the CM approach. The covalent bond (e.g. in H2) owes its energy to the configuration mix HfiH <— H H. A wave-function for the hydrogen molecule based on just one spin-paired form does not lead to a stable bond. Both spin forms are necessary. Addition of ionic configurations improves the bond further and in the case of heteroatomic bonds generates polar covalent bonds. 

One of the most convenient ways to picture the sharing of electrons between atoms in covalent or polar covalent bonds is to use electron-dot structures, or Lewis structures, named after G. N. Lewis of the University of California at Berkeley. An electron-dot structure represents an atom s valence electrons by dots and indicates by the placement of the dots how the valence electrons are distributed in a molecule. A hydrogen molecule, for example, is written showing a pair of dots between the hydrogen atoms, indicating that the hydrogens share the pair of electrons in a covalent bond ... 

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