Compounds 1 Forming Chemical Bonds

Chromium is able to use all of its >d and As electrons to form chemical bonds. It can also display formal oxidation states ranging from Cr(—II) to Cr(VI). The most common and thus most important oxidation states are Cr(II), Cr(III), and Cr(VI). Although most commercial applications have centered around Cr(VI) compounds, environmental concerns and regulations ia the early 1990s suggest that Cr(III) may become increasingly important, especially where the use of Cr(VI) demands reduction and incorporation as Cr(III) ia the product. 

Let s return for a last look at Pasteur s pioneering work. Pasteur took an optically inactive tartaric acid salt and found that he could crystallize from it two optically active forms having what we would now call the 2R,3R and 2S,3S configurations. But what was the optically inactive form he started with It couldn t have been meso-tartaric acid, because meso-tartaric acid is a different chemical compound and can t interconvert with the two chiral enantiomers without breaking and re-forming chemical bonds. 

Organic compounds make up more than 95% of all the chemical compounds known to exist. One reason for this is that carbon is unlike all other elements. It can form chemical bonds to connect (become bonded) with four other atoms. This ability to connect with other atoms (form bonds) is called valence. Carbon is said to have a valence of 4. The most unique feature of carbon is that it readily forms bonds with other carbon atoms to form what are usually called carbon chains. It also readily bonds to other elements, particularly hydrogen, oxygen, and nitrogen. 

I can say, for example, that it tends to form chemical bonds to five other atoms at a time, but can tolerate fewer and, at a push, more. It is a metal, probably quite a soft one, heavier than iron but lighter than lead. Many of its compounds - its combinations with other elements - will be coloured. It will be apt to form bonds to other niobium atoms - so-called metal-metal bonds. It will behave chemically in a similar manner to the element vanadium, but will be more similar still to tantalum. 

Silicon is chemically similar to carbon. An atom of silicon, like an atom of carbon, has four electrons in its outermost shell, and these electrons are available for forming chemical bonds. Like carbon, silicon can bond with four other elements to create a huge range of different compounds. This fact alone, some say, makes silicon a possible backbone for biological molecules. However, a more detailed examination raises doubts about the ability of silicon to form the kinds of structures necessary to build or sustain any form of life as we know it. 

Atoms of two or more different elements can form chemical bonds with each other to yield a product that is entirely different from the elements. Such a substance is called a chemical compound. 

First things first, you need to understand the nature of elements, and their oxidation states (number of bonds). Every single element is capable of forming chemical bonds with other elements (with the exception of a few noble gases ). The oxidation states are what determines how many bonds a particular element can form, and to what other elements. When elements combine, they form chemical compounds. All of the atoms within a chemical compound show specific oxidation states. Oxidation states are not really states, but definitions of bonding, which are dictated by each individual element. Each element can form any where from either 0 to 7 bonds. These numbers represent the number of bonds the element can form (look at a modem periodic table, such that included in the Merck Index —the oxidations states are written in the upper left comer of each element). These numbers clearly indicate the number of bonds each element is capable of forming. 

A further problem arises from the fact that catalysts Vork by forming chemical bonds with the substrate molecules. Thus metals form hydrides with H2 and H donors such as CH4 or NH3, or oxides with O2 and 0 donors such as CO2 or NO2. It is presumably these surface compounds that play the role of active intermediates in the catalytic reaction. Under slightly different conditions, however, it is possible to extend the catalyst-adsorbate reaction to produce bulk compounds, e.g., hydrides and oxides. In view of this ability of the surface phase to propagate into the bulk in many instances, it is not at all clear that only the surface of the solid catalyst is active in the reaction. Such complexities add enormously to the difficulty of interpreting the kinetic data from heterogeneously catalyzed reactions. 

As a group of typical metal elements, lanthanide elements can form chemical bonds with most nonmetal elements. Some low-valence lanthanide elements can form chemical bonds in organometallic or atom cluster compounds. Because lanthanide elements lack sufficient electrons and show a strong repulsive force towards a positive charge, chemical bonds between lanthanide metals have not yet been observed. Table 1.4 shows that 1391 structure-characterized lanthanide complexes were reported in publications between 1935 and 1995 and these are sorted by chemical bond type. 

Like helium, neon forms no compounds, but it is a very useful element. For example, neon is widely used in luminescent lighting (neon signs). Argon, which recently has been shown to form chemical bonds under special circumstances, is used to provide the noncorrosive atmosphere in incandescent light bulbs, which prolongs the life of the tungsten filament. 

Chemical posttreatment is already widely utilized and will probably be even more widely utilized in the future. It renders hydrophilic silica organophilic or hydrophobic by treating preferably pyrogenic silica, but also precipitated silicas, with organic or element-organic compounds, which react with the silanol groups forming chemical bonds ... 

In the present-day model of the atom, neutrons and protons form a nucleus at the center of the atom. Negatively-charged electrons are distributed in the space around the nucleus. The electrons with the most energy are farthest from the nucleus and occupy the outermost energy level. Recall from Chapter 2 that evidence for the existence of energy levels came from the interpretation of the emission spectra of atoms. It s important to know about energy levels in atoms because it helps explain how atoms form chemical bonds and why they form particular kinds of compounds, for example ionic or covalent compounds. 

Compounds are held together by chemical bonds, or links between atoms. Such links are known to arise from the deployment of the electrons of the outer shells of atoms, the so-called valence shells. Valence is a term signifying the power of atoms to form chemical bonds, and is derived from the Latin word for strength. Valete ( Be strong ) was what the Romans said on parting. We have seen that the electronic structure of atoms displays a periodicity that is captured by the layout of the kingdom, and we can expect that the ability to form bonds will show a similar periodicity. This general periodicity of number and type of bond is what we limit our discussion to here. 

Atoms combine to make compounds by forming chemical bonds. Several different types of chemical bonds are possible, and once we learn to recognize them, these types of bonds will help us to understand some of the chemical properties of many substances. 

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