Chemical Properties and Trends

The preparation and use of LiEt3BT and LiAlT4 at maximum specific activity (57.5 Cimmol ) has also been described.  

Hydrogen is a colourless, tasteless, odourless gas which has only low solubility in liquid solvents. It is comparatively unreactive at room temperature though it combines with fluorine even in the dark and readily reduces aqueous solutions of palladium(II) chloride  

An even more effective homogeneous hydrogenation catalyst is the complex [RhClfPPhsfs] which permits rapid reduction of alkenes, alkynes and other unsaturated compounds in benzene solution at 25°C and 1 atm pressure (p. 1134). The Haber process, which uses iron metal catalysts for the direct synthesis of ammonia from nitrogen and hydrogen at high temperatures and pressures, is a further example (p. 421). 

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Chemical reactivity and trends Table 22.1 Some properties of Group 5 elements... 

This section concentrates on physical and chemical properties, and deals mainly with aquo complexes. The intention is to focus on trends, particularly in relation to the Periodic Table, which will not be apparent in later volumes with their metal-oriented arrangement. Trends in stability constants and in kinetic properties, both substitution and redox, of aquo ions are covered in Chapters 7 and 9 in Volume 1. 

A modem periodic table usually shows the atomic number along with the element symbol. As you already know, the atomic nnmber also indicates the number of electrons in the atoms of an element. Electron confignrations of elements help to explain the recurrence of physical and chemical properties. The importance and usefulness of the periodic table lie in the fact that we can use our understanding of the general properties and trends within a group or a period to predict with considerable accuracy the properties of any element, even though that element may be unfamiliar to us. 

An additional important point that one should bear in mind is that the calculations describe explicitly only the isolated molecule in the gas phase (in some cases, simulation of solvation is attempted but such calculations are still relatively rare). It is now well established that in many cases solvation affects drastically a wide variety of molecular and chemical properties and could even reverse experimental trends, relative stabilities, etc61. This point should be kept is mind whenever comparing calculations with experimental data, particularly when charged or highly polar molecules are involved. For energies we use hartrees or kcalmol-1 throughout. 1 hartree = 2625.54 kJmol-1 = 627.52 kcal mol i. 

As researchers performed experiments that advanced them along the row of actinide elements on the Periodic Table, the general trends with increasing atomic number were smaller production probabilities expressed as cross sections (a consequence of the diminishing fission barrier and higher fission probabilities), an increased probability of decay by a-particle emission (a consequence of increasing a-decay Q values) and shorter half-lives. For the elements below fermium, spontaneous fission is not an important decay mode. Experimental work was dominated by radiochemical techniques in which atomic number was determined by chemical properties and atomic mass was determined by mass spectrometry and the connections of nuclei to one another by the processes of radioactive decay. The physical separation and detection methods that were used in later work were developed in the 1960s. 

The use of vitamins in humans consumes ca 40% of vitamins made worldwide. The majority of the vitamins, particularly in countries outside the United States, are used in animal husbandry. It is well estabUshed (21) that vitamins are critical to animal productivity, especially under confined, rapid growth conditions. Newer information (22) has shown that vitamin E added to catde feed has the additional effect of significantly prolonging beef shelf life in stores. Additional appHcations of vitamins exist. A small but growing market segment involves cosmetics (qv) (23). The use of the chemical properties of the vitamins, particularly as antioxidants (qv) in foods and, more recently, in plastics (vitamin E (24)), has emerged as a growing trend. 

General similarities and trends in the chemical properties of the elements had been noticed increasingly since the end of the eighteenth century and predated the observation of periodic variations in physical properties which were not noted until about 1868. However, it is more convenient to invert this order and to look at trends in atomic and physical properties first. 

Other periodic trends which occur in the chemical properties of the elements and which are discussed in more detail throughout later chapters are ... 

The alkali metals form a homogeneous group of extremely reactive elements which illustrate well the similarities and trends to be expected from the periodic classification, as discussed in Chapter 2. Their physical and chemical properties are readily interpreted in terms of their simple electronic configuration, ns, and for this reason they have been extensively studied by the full range of experimental and theoretical techniques. Compounds of sodium and potassium have been known from ancient times and both elements are essential for animal life. They are also major items of trade, commerce and chemical industry. Lithium was first recognized as a separate element at the beginning of the nineteenth eentury but did not assume major industrial importance until about 40 y ago. Rubidium and caesium are of considerable academic interest but so far have few industrial applications. Francium, the elusive element 87, has only fleeting existence in nature due to its very short radioactive half-life, and this delayed its discovery until 1939. 

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