Three important atomic properties

We now explore three important atomic properties the atomic (and ionic) radius, the ionization energy, rmd the electron affinity. These properties are of great significance in chemistry and biology, for they are controls on the number and types of chemical bonds the atom crm form. Indeed, we can use these properties to reveal an importcmt reason for the unique role of carbon in biology. 

A very useful thermodynamic cycle links three important physical properties homolytic bond dissociation energies (BDE), electron affinities (EA), and acidities. It has been used in the gas phase and solution to determine, sometimes with high accuracy, carbon acidities (Scheme 3.6). " For example, the BDE of methane has been established as 104.9 0.1 kcahmol " " and the EA of the methyl radical, 1.8 0.7 kcal/mol, has been determined with high accuracy by photoelectron spectroscopy (PES) on the methyl anion (i.e., electron binding energy measurements). Of course, the ionization potential of the hydrogen atom is well established, 313.6 kcal/ mol, and as a result, a gas-phase acidity (A//acid) of 416.7 0.7 kcal/mol has been... 

Many molecules are obtained and used in a crystalline form, the nature of which can have e significant impact on their properties and behaviour. Moreover, it is sometimes possible foi a given material to exist in more than one crystalline form, depending upon the conditions under which it was prepared. This is the phenomenon of polymorphism. This can be important because the various polymorphs may themselves have different properties. It is Iberefore of interest to be able to predict the three-dimensional atomic structure(s) that a gi en molecule may adopt, for those cases where it is difficult to obtain experimental data and also where one might wish to prioritise molecules not yet synthesised. 

Table 27.1 lists some of the important atomic and physical properties of these three elements. The prevalence of naturally occurring isotopes in this triad limits the precision of their quoted atomic weights, though the value for Ni was improved by more than two orders of magnitude in 1989... 

In order to understand the relationship between the properties of a material and its structure, which is the raison d etre of the materials scientist, three important experimental areas of investigation may be necessary. Firstly, of course, the physical or mechanical properties in question must be measured with maximum precision, then the structure of the material must be characterised (this itself may refer to the atomic arrangement or crystal structure, the microstructure, which refers to the size and arrangement of the crystals, or the molecular structure). Finally, the chemical composition of the material may need to be known. 

Table VIII lists some of the more important physical properties of TeF4. In the orthorhombic crystals, each tellurium atom is surrounded by three terminal and two bridging fluorine atoms, arranged at the apices of a distorted square pyramid. The square-pyramidal units are linked by cis-bridging atoms into endless chains with a bridge angle of 159°. The nearest intermolecular contacts to the tellurium atom are 2.94 and 3.10 A, so that there are no other significant interactions. This geometry is in accordance with the steric activity of the lone electron pair at the tellurium atom. Figure 6 shows the atomic arrangement (54).

We have seen that isotope effects on the properties of atoms and molecules are usually small, and this is true for all except the lightest elements. Consequently separation of single isotopes from mixtures of isotopes or isotopomers is tedious and difficult. The difficulty is compounded by the fact that the desired isotope is often present at low or very low concentration in the starting material (normally a naturally occurring fluid, ore, or mineral). Even so, the nuclear properties of certain separated isotopes are enough different from their sisters to justify the (usually enormous) expense of preparing isotopically pure or nearly pure materials. Three important examples follow ... 

We turn now to a very important class of materials that have the formula /IBC3, with the C frequently oxygen. Strontium titanate is a familiar example and one we shall use for illustrative purposes. Titanium is in the D4 column of the Solid State Table, having four electrons beyond its argonlike core. Strontium has two electrons outside its kryptonlike core, so we may think of the six valence electrons as having been transferred to the three oxygen atoms to form a simple ionic system. As we shall see, however, the titanium d states form the lowest conduction band and are important in the bonding properties as well. 

Two or more atoms of the same element that contain different numbers of neutrons are said to be isotopes. An atom of a specific isotope is called a nuclide. Isotopes have similar chemical properties. Hydrogen has three important isotopes H (pro-tium), 2H (deuterium), and 3H tritium). 99.98% of naturally occurring hydrogen is protium. Examples of three isotopes for carbon are ... 

Particles less than about 1 /btm (10" cm) in diameter have a significant percentage of their atoms at particle surfaces. At such sizes and smaller (in the colloidal size range), particles have important surface properties, whereas larger particles generally do not. Such surface properties have at least three significant consequences ... 

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