Ionic bond, electronegativity

Valence Electrons Covalent and Ionic Bonds Electronegativity Polar Covalent... 

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 ... 

As opposed to the oxides, fluoride compounds are characterized by the formation of mostly ionic bonds. This peculiarity is related to fluorine s high electronegativity. 

Ionic Bonding, Partial Ionic Character, and Electronegativity... 

Chapter 2. Ionic Bonding, Partial Ionic Character, and Electronegativity Papers SP 23 to SP 27 255... 

When the difference in electronegativities is great, the orbital may be so far over to one side that it barely covers the other nucleus. This is an ionic bond, which is seen to arise naturally out of the previous discussion, leaving us with basically only one type of bond in organic molecules. Most bonds can be considered intermediate between ionic and covalent. We speak of percent ionic character of a bond, which indicates the extent of electron-cloud distortion. There is a continuous gradation from ionic to covalent bonds. 

Whether a carbon-metal bond is ionic or polar-covalent is determined chiefly by the electronegativity of the metal and the structure of the organic part of the molecule. Ionic bonds become more likely as the negative charge on the metalbearing carbon is decreased by resonance or field effects. Thus the sodium salt of acetoacetic ester has a more ionic carbon-sodium bond than methylsodium. 

Atoms in a molecule are joined by bonds. Bonds are formed when the valence or outermost electrons of two or more atoms interact. The nature of the bond between atoms goes a long way toward determining the properties of the molecule. Chapter 5 introduced the two common types of chemical bonds covalent and ionic. Elements with similar electronegativities share electrons and form covalent bonds. But elements with greatly different electronegativities exchange one or more electrons. This is called an ionic bond. 

Covalent bonds form between atoms with similar electronegativities. In these reactions, electrons do not migrate from one atom to another as they do in ionic bonds they are shared by the atoms in the molecule. A good way to visualize this was proposed by Gilbert Lewis, a chemist at the University of California, Berkeley. His representations of molecular bonds are called Lewis dot structures. These structures use dots to denote the valence electrons of an element or molecule. 

The difference in electronegativity between sodium and chlorine and between hydrogen and oxygen causes one pair of atoms to form an ionic bond and the other pair to form a covalent bond. 

The electronegativity of sodium and chlorine differ by 2.23, whereas the difference between hydrogen and oxygen is only 1.24 (see Table 7.1). As a general rule, molecules made up of two atoms with electronegativity differences greater than 2.0 form ionic bonds. Molecules whose atoms have electronegativity differences of less than 2.0 form covalent bonds. Ionic-bonded salt and covalent-bonded water conform to that rule. 

Chromium has a similar electron configuration to Cu, because both have an outer electronic orbit of 4s. Since Cr3+, the most stable form, has a similar ionic radius (0.64 A0) to Mg (0.65 A0), it is possible that Cr3+ could readily substitute for Mg in silicates. Chromium has a lower electronegativity (1.6) than Cu2+ (2.0) and Ni (1.8). It is assumed that when substitution in an ionic crystal is possible, the element having a lower electronegativity will be preferred because of its ability to form a more ionic bond (McBride, 1981). Since chromium has an ionic radius similar to trivalent Fe (0.65°A), it can also substitute for Fe3+ in iron oxides. This may explain the observations (Han and Banin, 1997, 1999 Han et al., 2001a, c) that the native Cr in arid soils is mostly and strongly bound in the clay mineral structure and iron oxides compared to other heavy metals studied. On the other hand, humic acids have a high affinity with Cr (III) similar to Cu (Adriano, 1986). The chromium in most soils probably occurs as Cr (III) (Adriano, 1986). The chromium (III) in soils, especially when bound to... 

In some cases, elements having electronegativities too low to give ionic bonding with hydrogen also tend to be unreactive, so that direct combination of the elements is not feasible. In such cases, the procedure just described can be used to prepare the hydride. For example, silicon hydride, SiH4 (known as silane), can be produced by the reactions... 

An ionic bond is normally formed between two elements of very different electronegativity (typically greater than 1.5). 

Measures of bond ionicity and electronegativity Let us now discuss the bond polarities and charge distributions in greater detail. For a general hydride bond NBO of the form... 

Generally, expect an ionic bond whenever you have a metal (Cs) bonded to a nonmetal (I). If you have a table of electronegativities to refer to, the electronegativity difference is greater than 1.7 for ionic compounds. 

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