Bond, chemical types covalent

Chemically, proteins are unbranched polymers of amino acids linked head to tail, from carboxyl group to amino group, through formation of covalent peptide bonds, a type of amide linkage (Figure 5.1). 

Two types of chemical bonds, ionic and covalent, are found in chemical compounds. An ionic bond results from the transfer of valence electrons from the atom of an electropositive element (M) to the atom(s) of an electronegative element (X). It is due to coulombic (electrostatic) attraction between the oppositely charged ions, M (cation) and X (anion). Such ionic bonds are typical of the stable salts formed by combination of the metallic elements (Na, K, Li, Mg, etc.) with the nonmetallic elements (F, Cl, Br, etc.). As an example, the formation of the magnesium chloride molecule from its elemental atoms is shown by the following sequence ... 

Physical hardness can be defined to be proportional, and sometimes equal, to the chemical hardness (Parr and Yang, 1989). The relationship between the two types of hardness depends on the type of chemical bonding. For simple metals, where the bonding is nonlocal, the bulk modulus is proportional to the chemical hardness density. The same is true for non-local ionic bonding. However, for covalent crystals, where the bonding is local, the bulk moduli may be less appropriate measures of stability than the octahedral shear moduli. In this case, it is also found that the indentation hardness—and therefore the Mohs scratch hardness—are monotonic functions of the chemical hardness density. 

The examples cited in this chapter are but a rather small and arbitrary selection from the richly varied possibilities for supramolecular bonding. Recognition of the intrinsic chemical (partially covalent, exchange-type) character of supramolecular interactions leads inevitably to an extended definition of chemistry that includes many aspects of nanoscale aggregation, structure, and function in the biophysical and material-science domains. From this viewpoint, the molecule is seen to be... 

In Sections 9-3 and 9-4, we will show you two types of chemical bonds ionic and covalent. It is important to be able to represent compounds in terms of the atoms and valence electrons that make up the chemical species (compounds or polyatomic ions). One of the best ways is to use Lewis symbols and structures. 

Non-covalent bonding includes hydrogen bonding, ionic bonds, or hydrophobic bonds. These types of bonding are involved in binding of chemicals to plasma proteins. They could also underlie the interaction between a chemical and a receptor or enzyme. Thus, the interaction between TCDD and the Ah receptor (AhR) and the intercalation of doxorubicin in DNA involve non-covalent bonds. 

In this chapter, we explored two types of chemical bonds ionic and covalent. Ionic bonds are formed when one or more electrons move from one atom to another. In this way, the atoms become ions—one positive, the other negative—and are held together by the resulting electrical attraction. Covalent bonds form when atoms share electrons. When the sharing is completely equitable, the bond is nonpolar covalent. When one atom pulls more strongly on the electrons because of its greater electronegativity, the bond is polar covalent and a dipole may be formed. 

Before considering the structures of molecules, we must begin with a discussion of chemical bonds, the forces that hold atoms together in molecules. There are two types of chemical bonds, the ionic bond and the covalent bond. 

In these compounds the metal-carbon bonds are highly covalent and chemically rather stable. The chemical reactivity of the trivalent compounds, in particular those of the type R3M is associated with an easy oxidizability to the pentavalent state rather than with rupture of metal-carbon bonds. Thus, the vigorous reaction of the trialkyl compounds R3M with air depends on the following oxidative transformation ... 

Chemical bonds in inorganic chemistry are not limited to covalency. Very important are the dative donor-acceptor) bonds. Unlike normal covalent bonds, which are formed by pairing of electrons with a one-electron contribution from each atom, dative bonds are two-electron bonds formed by donation of an electron pair from one atom to another. There is a general tendency to assume that the two-electron bonds between a certain pair of atoms e.g. boron and nitrogen in H2B-NH2 and HsB- NH3) are identical, regardless of the origin of electrons (i.e. covalent and dative), but it has been pointed out that a distinction between the two types should be made. ... 

The forces that hold atoms together in compounds are called chemical bonds. One way that atoms can form bonds is by sharing electrons. These bonds are called covalent bonds, and the resulting collection of atoms is called a molecule. Molecules can be represented in several different ways. The simplest method is the chemical formula, in which the symbols for the elements are used to indicate the types of atoms present, and subscripts are used to indicate the relative numbers of atoms. For example, the formula for carbon dioxide is C02, meaning, of course, that each molecule contains 1 atom of carbon and 2 atoms of oxygen. 

Molecular bonds are formed from the electrostatic interactions between electrons and the nuclei of atoms. There are many different electronic arrangements that lead to bond formation and the type of bond formed influences the properties of the compound that results. It is the outermost electrons of an atom that are involved in bond formation. The archetypical chemical bond is the covalent bond and we can probably best imagine this as formed from outer electrons shared between two atoms. Take the example of two fluorine atoms that form the fluorine molecule ... 

Electronegativity is a measure of the ability of an atom or molecule to attract electrons in the context of a chemical bond. The type of bond formed is largely determined by the difference in electronegativity between the atoms involved. Atoms with similar electronegativities will share an electron with each other and form a covalent bond. However, if the difference is too great, the electron will be permanently transferred to one atom and an ionic bond will form. Furthermore, in a covalent bond if one atom pulls slightly harder than the other, a polar covalent bond will form. 

The interatomic forces between atoms result in an atomic aggregate with sufficient stability to form chemical bonds within a molecule. According to the valence bond theory, a chemical bond is formed when an electron in one atomic orbital pairs its spin with that of an electron supplied by another atomic orbital, these electrons are then shared between two or more atoms so that the discrete nature of the atom is lost. Three main types of chemical bond are considered covalent, electrostatic (ionic) and metallic bonds. 

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