Ionic versus covalent bonds

The concepts which we need for understanding the structural trends within covalently bonded solids are most easily introduced by first considering the much simpler system of diatomic molecules. They are well described within the molecular orbital (MO) framework that is based on the overlapping of atomic wave functions. This picture, therefore, makes direct contact with the properties of the individual free atoms which we discussed in the previous chapter, in particular the atomic energy levels and angular character of the valence orbitals. We will see that ubiquitous quantum mechanical concepts such as the covalent bond, overlap repulsion, hybrid orbitals, and the relative degree of covalency versus ionicity all arise naturally from solutions of the one-electron Schrodinger equation for diatomic molecules such as H2, N2, and LiH. 

Can an absolute description of covalency versus ionicity be obtained from observable properties First, bond type is generally defined in principle in terms of the total electron density, since the molecular energy is a unique functional of the electron density (Hohenberg and Kohn, 1964). From x-ray diffraction, total densities are available, but the problem is to determine which aspects of the data are most informative. We can inspect the topology of the total density in the way described by Bader and Ngu-... 

Use SpartanView to compare bond-density surfaces of methanol (CH3OH) and sodium methoxide (CH30Na). Which bond has higher electron density between the nuclei, 0-H or O-Na Repeat for acetic acid (CH3CO2H) and sodium acetate (CH3C02Na), and for hydrogen chloride (HCl) and sodium chloride (NaCl). How do your results relate to the covalent versus ionic nature of the bonds ... 

COVALENT VERSUS IONIC METAL-LIGAND BONDS... 

Distinguish between the terms electronegativity versus electron affinity, covalent bond versus ionic bond, and pure covalent bond versus polar covalent bond. Characterize the types of bonds in terms of electronegativity difference. Energetically, why do ionic and covalent bonds form ... 

The gas in equilibrium with the adsorbate Enhancement of the amount of gas molecules on the surface of a solid caused by covalent or ionic bonding A plot of amount adsorbed versus -ln(-ln(P/Pj))... 

In conclusion, several semiconductors exhibit the same behavior as sihcon under very high stress. This could be a general feature for all semiconducting materials, but with specificities for each material depending on the structure and the covalent versus ionicity balance of the bonding. 

Distinguish between the terms electronegativity versus electron affinity, covalent bond versus ionic bond, and pure covalent bond versus polar covalent bond. 

The problem of recovery leads to the question of hardness. Hard substances have a high number of strongly directed, covalent chemical bonds per unit volume. Soft substances generally have fewer bonds per unit volume or bonds that are weak or weakly directed, such as ionic or dipole attractive forces. Bond energy per unit volume has the same dimensions as pressure (force per unit area), and a plot of hardness measured by the Knoop indenter versus the bond energy per molar volume for various substances is essentially linear, provided that one chooses substances for which the bonding is predominantly of one type (i.e., not mixed, as in graphite or talc). 

Ionic Versus Covalent Character and Bond Polarities... 

Figure 3.5 Plot of the association constant of some 1 1 metal cation-hydroxy complexes at zero ionic strength (see Chap. 4) versus the electrostatic function luZon/irti + Toh). where the association reaction is written Af + OH"=A/OH " , and z and r are the charge and radius in nanometers (nm) or angstroms (A) (1 nm = 1 A) of cation M and OH ( oh = 1-40 nm). Cation radii are from Shannon and Prewitt (1969), log values from Baes and Mesmer (1981). The slope of the straight line suggests the contribution of electrostatic (ionic) bonding to the stability of the complexes. The extent to which species plot above this line presumably reflects the increased contribution of covalency to their stabilities.

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