Atomic structure polarized covalent

Formal charges are based on Lewis structures m which electrons are considered to be shared equally between covalently bonded atoms Actually polarization of N—H bonds m ammonium ion and of B—H bonds m borohydride leads to some transfer of positive and negative charge respectively to the hydrogens... 

In 1947 the present writer published a paper [1] in which he studied the interaction between the polar covalent diatomic molecule A—X and the atom B which has a complete electronic shell. The atom X, in particular, may be a hydrogen atom. To simplify the problem only four electrons were considered two electrons from the bond A—H, and the unshared pair of electrons from atom B, viz. A H B. The respective problem was solved approximately by the method of valence structures. The following structures were adopted ... 

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 ... 

It is known that the chemistry of enolates depends on the nature of the metal. Moreover, the metals are an integral part of the structures of enolates. Lithium enolates are most frequently employed, and in the solid state the lithium cations definitely are associated with the heteroatoms rather than with the carbanionic C atoms. Presumably the same is true in solution. The bonding between the heteroatom and the lithium may be regarded as ionic or polar covalent. However, the heteroatom is not the only bonding partner of the lithium cation irrespective of the nature of the bond between lithium and the heteroatom ... 

You have seen how Lewis structures can help you draw models of ionic, covalent, and polar covalent compounds. When you draw a Lewis structure, you can count how many electrons are needed by each atom to achieve a stable octet. Thus, you can find out the ratio in which the atoms combine. Once you know the ratio of the atoms, you can write the chemical formula of the compound. Drawing Lewis structures can become overwhelming, however, when you are dealing with large molecules. Is there a faster and easier method for writing chemical formulas ... 

The electronic structure of the boron atom is 1 s2 2s2 Ip1. It might be expected that boron would lose the outer electrons and be present in compounds as B3+ ions. This ionization, however, requires more than 6700 kJ mol-1, and this amount of energy precludes compounds that are strictly ionic. Polar covalent bonds are much more likely, and the hybridization can be pictured as follows. Promoting a 2s electron to one of the vacant 2p orbitals can be accomplished followed by the hybridization to produce a set of sp2 hybrid orbitals ... 

Fundamental adhesion is connected with the nature of the bonds producing cohesion between two media. These bonds may be classified into two categories, namely strong bonds (polar, covalent and metallic bonds) and secondary bonds (hydrogenous and Van der Waals bonds). Different atomic or molecular models have been proposed to describe the electronic structure of interfaces. None however, is sufficient for calculating the intensity of adhesion forces for systems of practical interest. 

Hydrogen bromide, HBr, is used to make pharmaceuticals that require bromine in their structure. Each hydrogen bromide molecule has one hydrogen atom bonded to one bromine atom by a polar covalent bond. The bromine atom attracts electrons more than does the hydrogen atom. Draw a rough sketch of the electron-cloud that represents the electrons involved in the bond. 

Ionic bonds are described as well. The transition from covalent over polar covalent to ionic bonds is veiy fluent and depends on the difference in electronegativity between the atoms. The covalent bonds consist of sharing an electron pair and ionic bonds are electrostatic interactions between a cation and an anion. Solid ionic compounds are often arranged in lattice structures with many similarities with the lattice structures that we saw for the metallic compounds. The type of lattice structure for solid ionic compound depends on the ration between the radius of the cation and anion. 

Draw an appropriate covalent Lewis structure formed by the simplest combination of atoms in Problem 4.63 for each solution that involves a nonpolar or polar covalent bond. 

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