Atoms covalent systems

In systems where atoms can change their hybridization and coordination, one should take into account the dependence of the bond energy on the presence and orientation of other bonds (that is, on the bond order). For covalent systems, such a dependence was suggested by Abell [64]... 

Both ionic and covalent bonds involve valence electrons, the electrons in the outermost energy level of an atom. In 1920, G. N. Lewis, the American chemist shown in Figure 9, came up with a system to represent the valence electrons of an atom. This system—known as electron-dot diagrams or Lewis structures —uses dots to represent valence electrons. Lewis s system is a valuable model for covalent bonding. However, these diagrams do not show the actual locations of the valence electrons. They are models that help you to keep track of valence electrons. 

You know that many atoms covalently bond to form molecules that behave as a single unit. These units can be represented by chemical formulas and names that are used to identify them. When naming molecules, the system of rules is similar to the one you used to name ionic compounds. 

Our discussion in the previous part of this section showed some of the variety associated with surface reconstructions on the surfaces of metals with both the fee and bcc structures. Just as such surfaces have served as a proving ground for our understanding of metals, semiconductor surface reconstructions have played a similar role in the study of covalent systems. The starting point for the analysis in this section is a synthesis of a wide variety of experimental data in the form of a phase diagram for the Si surfaces. The objective in the remainder of the section will be to try to rationalize, even predict, such phases on the basis of microscopic analysis. Beyond their academic interest to surface science, the reconstructions in Si also impact the nature of the growth processes that occur when atoms are deposited on exposed surfaces. 

The quantum mechanical many-body nature of the interatomic forces is taken into account naturally through the Hellmann-Feynman theorem. Since the scheme usually uses a minimal basis set for the electronic structure calculation and the Hamiltonian matrix elements are parametrized, large numbers of atoms can be tackled within the present computer capabilities. One of the distinctive features of this scheme in comparison with other empirical schemes is that all the parameters in the model can be obtained theoretically. It is therefore very useful for studying novel materials where experimental data are not readily available. The scheme has been demonstrated to be a powerful method for studying various structural, dynamical, and electronic properties of covalent systems. 

Two atoms covalently bonded form a diatomic molecule. A covalent bond can be interpreted as a shearing of electrons by both atoms. The diatomic molecule is a much simpler system than a solid and is well described within the molecular orbital (MO) framework, which is based on the concept of overlapping of atomic eigenfunctions [3]. 

COMPOUNDS CONTAINING a halogen atom covalently bonded to an sp hybridized carbon atom are named haloalkanes or, in the common system of nomenciature, alkyl halldes.Jhe general symbol for an alkyl halide is R—X, where X may be F, Ci, Br, or i ... 

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