Cohesive forces Covalent

While sharing of electrons, i.e., covalent bonding, is the major component of the cohesive force in intermetallics, rationalization of their structure formation based on such chemical bonding is not trivial, because of the failure of the common electron counting rules that chemists have developed over the years from the studies of covalent compounds. The origin of the problem is the well-delo-... 

After intimate contact is achieved between adhesive and adherend through wetting, it is believed that permanent adhesion results primarily through forces of molecular attraction. Four general types of chemical bonds are recognized as being involved in adhesion and cohesion electrostatic, covalent, and metallic, which are referred to as primary bonds, and van der Walls forces, which are referred to as secondary bonds. 

Although the cohesive forces in such an idealized metal as described would be nondirectional (as in ionic solids), the orientation effects of d orbitals contribute a directional-covalent component to the bonding in transition metals that requires a more sophisticated definition for metallic bonding. The intemuclear distances in the close packed, or nearly close packed, stmcmres of most metalhc elements ate small enough that the valence orbitals on the metal atoms can overlap (in the valence-bond model) or combine to form COs (in the MO or Bloch model). 

In the transition metals, for example, iron, the core orbitals (Is, 2s, 2p, 3s, and 3p) form narrow bands, which are all filled. The 3d, 4s, and 4p bands form wide bands. At the appropriate interatomic distance for iron metal, there is an overlap between the 3d and the 4s-4p orbitals on different sites. There is thus a set of orbitals of mixed 3d, 4s, and 4p character that forms a continuous band of orbital energies. This band is occupied only by the eight valence electrons per Fe site. The most antibonding orbitals of this valence band are empty, and this explains the cohesive forces of metals. The bonds, referred to as metal bonds, are type of covalent bond. 

The values may be understood in terms of the cohesive forces operating in the various cases. The basis of the theory is that which is presented in Section 4.3 in the discussion of the atomic volume curve. The forces range from metallic bonding in Groups 1 to 12/13, followed by covalent bonding in the p-block non-metallic elements, to van der Waals forces (London interatomic forces) which operate alone in the Group 18 elements. 

The cohesion force of an adhesive is huilt up from a wide spectrum of forces including molecular dipole, hydrogen-hridge and covalent and other cohesion forces and is a mixture of these in varying proportions. 

Diamond is an example of an atomic solid in which the cohesive forces that hold the solid together are actually covalent chemical bonds. These forces are extremely strong and result in melting points of thousands of degrees. 

Film and coating fonnation occurs when biopolymer molecules interact through cohesive forces, named H-bonding, ionic bonds and covalent bonds (disulfide bonds). Factors affecting film strength are the chemical nature of the biopolymer and the rest of the components of the formulation (plasticizer type and amount and food additives), and the film forming process. 

The weaker secondary bonding mechanisms generally play a lesser role in the presence of primary bonds such as providing the attractive forces between covalently bonded chains in a polymer. However, they provide the cohesive forces that hold materials together when no primary bonds are present. All of these secondary bonds involve dipole-dipole interactions. 

Two species combine to form a complex in water if the sum of the intermolecular forces between them more than olfsets the sum of the loss of favorable interactions with solvent and any unfavorable interactions that develop between solutes during complex formation. Collectively the interactions between non-bonded species are referred to as cohesive forces, defined as those forces lost when the species are transferred to infinite separation in the gas phase. While it is common to classify chemical forces as covalent or non-covalent, the interactions are fundamentally the same only the magnitude of the interactions varies. Cohesive, non-specific forces are weak compared to covalent interactions typically we consider cohesive forces as those forces with strengths less than 1% of covalent bond strengths. We will see, however, that this definition is somewhat arbitrary and in fact a continuum of interaction energies exists. 

If the concentration of junction points is high enough, even branches will contain branches. Eventually a point is reached at which the amount of branching is so extensive that the polymer molecule becomes a giant three-dimensional network. When this condition is achieved, the molecule is said to be cross-linked. In this case, an entire macroscopic object may be considered to consist of essentially one molecule. The forces which give cohesiveness to such a body are covalent bonds, not intermolecular forces. Accordingly, the mechanical behavior of cross-linked bodies is much different from those without cross-linking. 

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