Binary covalent compounds bonding

Nearly all binary molecular compormds involve two nonmetals bonded together. Although many nonmetals can exhibit different oxidation numbers, their oxidation numbers are not properly indicated by Roman numerals or suffixes. Instead, elemental proportions in binary covalent compounds are indicated by using a prefix system for both elements. The Greek and Latin prefixes for one through ten are mono, di, tri, tetra, penta, hexa, hepta, octa, nona, and deca. The prefix mono- is omitted for both elements except in the common name for CO, carbon monoxide. We use the minimum number of prefixes needed to name a compound unambiguously. The final a in a prefix is omitted when the nonmetal stem begins with the letter o we write heptoxide, not heptaoxide. ... 

This is one of the binary covalent compounds that do not require prefixes. Iodine usually forms one bond, and hydrogen always forms one bond, so hydrogen iodide is HI. 

Two types of acids are named differently. Water solutions of binary covalent compounds containing hydrogen and a nonmetal are named following the pattern hydro(stem)ic acid, where (stem) is the stem of the name of the nonmetal bonded to hydrogen. Acids in which hydrogen is bonded to polyatomic ions have names based on the name of the polyatomic ion to which hydrogen is bonded. 

A large number of binary AB compounds formed by elements of groups IIIA and VA or IIA and VIA (the so-called III-V and II-VI compounds) also fcrystallize in diamond-like structures. Among the I-VII compounds, copper (I) halides and Agl crystallize in this structure. Unlike in diamond, the bonds in such binary compounds are not entirely covalent because of the difference in electronegativity between the constituent atoms. This can be understood in terms of the fractional ionic character or ionicity of bonds in these crystals. 

Solution The first two ions are single atom anions and are called sulfide and nitride. The next two are binary ionic compounds, calcium fluoride and potassium sulfide. The polyatomic ions hydroxide and sulfate are present in sodium hydroxide and sodium sulfate. Finally, the last two compounds are covalently bonded and are called sulfur trioxide and carbon tetrachloride. 

There is a variety of disparate binary oxide compounds. The change of physical properties is attributable to the range of bond types from essentially ionic to essentially covalent. 

The atomic and ionic properties of an element, particularly IE, ionic radius and electronegativity, underly its chemical behaviour and determine the types of compound it can form. The simplest type of compound an element can form is a binary compound, one in which it is combined with only one other element. The transition elements form binary compounds with a wide variety of non-metals, and the stoichiometries of these compounds will depend upon the thermodynamics of the compound-forming process. Binary oxides, fluorides and chlorides of the transition elements reveal the oxidation states available to them and, to some extent, reflect trends in IE values. However, the lEs of the transition elements are by no means the only contributors to the thermodynamics of compound formation. Other factors such as lattice enthalpy and the extent of covalency in bonding are important. In this chapter some examples of binary transition element compounds will be used to reveal the factors which determine the stoichiometry of compounds. 

The anesthetic dinitrogen oxide (N2O), commonly known as nitrous oxide, is a covalently bonded compound. Because it contains only two different elements, it is a binary molecular compound. Binary molecular compounds are composed of two different nonmetals and do not contain metals or ions. Although many of these compounds have common names, they also have scientific names that reveal their composition. Use the following simple rules to name binary molecular compounds. 

These ions with their opposite charges attract each other in the same way as do the simple ions in binary ionic compounds. However, the individual polyatomic ions are held together by covalent bonds, with all of the atoms behaving as a unit. For example, in the ammonium ion, NH +, there are four N—H covalent bonds. Likewise, the nitrate ion, N03, contains three covalent N—O bonds. Thus, although ammonium nitrate is an ionic compound because it contains the NH " and N03 ions, it also contains covalent bonds in the individual polyatomic ions. When ammonium nitrate is dissolved in water, it behaves as a strong electrolyte like the binary ionic compounds sodium chloride and potassium bromide. As we saw in Chapter 8, this occurs because when an ionic solid dissolves, the ions are freed to move independently and can conduct an electric current. 

Burger and co-workers 24, 25, 26) prepared a number of binary metal compounds with the very bulky silazane ligand N(SiMe3)2, e,g, MLa (M = Al, Ga, Cr, and Fe), ML2 (M = Be, Mn, Co, Ni, Zn, Cd, and Hg), ML (M = Li, Na, and Cu). It is reasonably certain that the ML2 compounds contain two-coordinated metals and that the metals are three-coordinated in ML3 compounds. Recently Bradley and co-workers isolated some additional three-coordinated species of the transition metals ML3 (M = Sc, Ti, and V) 27, 28, 29) and the lanthanides 30, 31), X-ray single-crystal diflFraction analysis revealed that the three-coordinated compounds exhibit either trigonal planar MN3 units (Ti, V, Cr, Fe, Al, and Ga) or pyramidal MN3 (Sc, Eu, and Yb) 32, 33, 34, 35), The reason for this structural difference is not yet clear, but it does appear that the more covalently bonded compounds have the trigonal planar configuration. 

It has been shown that not only the elemental semiconductors of group IV and binary compounds which are their analogs crystallize in tetrahedral structures in fact, a whole series of ternary compounds of various types with an average of four valence electrons per atom have the same property. Thus, the formation of covalent bonds based on the sp hybrids is not peculiar to elemental semiconductors and binary semiconducting compounds, but is also found in ternary semiconducting compounds. 

The covalent bond has localized electrons, and has little or no net charge transfer from one atom to the other. Silicon and diamond are prototypes of solids having covalent bonds, and the bonds in hydrogen and nitrogen molecules are also purely covalent. These bonds are oriented in space with respect to each other. Four-coordination (four nearest-neighbor atoms) often indicates covalency. Examples of covalent bonds in binary compounds are found in the solid silicon carbide (SiC) and nitrogen chloride molecules (NCI3). 

FIGURE 4.76 The schematic representation of the triangle of ionic (I), covalent (C) and metallic (M) tendencies of chemical bond, for the binary AB compounds, in terms of the differences of electronegativity paradigm after (Ketelaar, 1958 Putz, 2006). 

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