Atoms and ions 121

Ar, Cs, Ga or other elements with energies between 0.5 and 10 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection of atoms, ions and multi-atomic clusters (figure Bl.25.8). In SIMS, secondary ions (positive or negative) are detected directly with a mass spectrometer. 

In a plasma, the constituent atoms, ions, and electrons are made to move faster by an electromagnetic field and not by application of heat externally or through combustion processes. Nevertheless, the result is the same as if the plasma had been heated externally the constituent atoms, ions, and electrons are made to move faster and faster, eventually reaching a distribution of kinetic energies that would be characteristic of the Boltzmann equation applied to a gas that had been... 

Since detailed chemical structure information is not usually required from isotope ratio measurements, it is possible to vaporize samples by simply pyrolyzing them. For this purpose, the sample can be placed on a tungsten, rhenium, or platinum wire and heated strongly in vacuum by passing an electric current through the wire. This is thermal or surface ionization (TI). Alternatively, a small electric furnace can be used when removal of solvent from a dilute solution is desirable before vaporization of residual solute. Again, a wide variety of mass analyzers can be used to measure m/z values of atomic ions and their relative abundances. 

Let us consider next the mutual electrostatic energy of an atomic ion and one water molecule in a vacuum. The interaction will have its greatest value when the two particles are in contact and when the axis of the dipole lies along the line joining them, as shown in Figs. 22a and 22b. 

In Sec. 25 we examined the force of attraction between an atomic ion and a solvent dipole, and compared it with the attraction between two solvent dipoles. If we discuss water molecules in contact with the potassium ion K+, for example, we may expect to find that the function... 

A criterion for the presence of associated ion pairs was suggested by Bjerrum. This at first appeared to be somewhat arbitrary. An investigation by Fuoss,2 however, threw light on the details of the problem and set up a criterion that was the same as that suggested by Bjerrum. According to this criterion, atomic ions and small molecular ions will not behave as strong electrolytes in any solvent that has a dielectric constant less than about 40. Furthermore, di-divalent solutes will not behave as strong electrolytes even in aqueous solution.2 Both these predictions are borne out by the experimental data. 

Properties of Different Solvents. In discussing molecular dipoles in Sec. 25, we estimated the force of attraction between an atomic ion and a dipole having the most favorable orientation and found this attraction to be very strong. In any ionic co-sphere those molecular dipoles which have a favorable orientation will bo attracted, while those that have the opposite orientation will be repelled. Since the former are more numerous the solvent in the co-sphere is, on the whole, attracted toward the ion. Since the liquid is not incompressible, we must expect that this will lead to a contraction in each co-sphere. In any ionic solution the sum of the contractions that have taken place in the co-spheres of the positive and negative ions will be apparent if we measure accurately the volume of the solution. 

IUPAC (1988) Names of Hydrogen Atoms, Ions, and Groups, and for Reactions Involving Them. 

Chemists report numbers of atoms, ions, and molecules in terms of a unit called a mole. A mole is the analog of the wholesaler s dozen. A dozen could be defined as the number of soda cans in a twelve pack carton supplied by a wholesaler. Even if you could not open the carton to count the number of cans inside, you could find out how many cans are in a dozen by weighing the carton and dividing the mass of the carton by the mass of one can. A similar approach is used to define a mole (abbreviated mol) ... 

Because the mole tells us the number of atoms in a sample, it follows that 1 mol of atoms of any element is 6.0221 X 1023 atoms of the element (Fig. E.2). The same is true of 1 mol of any objects, including atoms, ions, and molecules ... 

The arrangement of atoms, ions, and molecules within a crystal is determined by x-ray diffraction (Major Technique 3, which follows this chapter), one of the most useful techniques for determining the structures of solids. 

When students do make an attempt to relate between the three levels of representation, several unexpected trends in their reasoning are revealed. In the majority of explanations given by students about chemical reactions in a review imdertaken by Andersson (1986), there was a clear extrapolation of physical attributes and changes from the macroscopic world to the particle or submicroscopic one. So, when wood bums, wood molecules are also said to bum. If metallic copper is bright reddish-brown, atoms of copper are also imagined to be reddish-brown in colour (Ben-Zvi, Eylon, Silberstein, 1986). One reason for such extrapolation of physical attributes of substances to the particulate level is the tendency of students to assume that the atoms, ions and molecules in a substance are veiy small portions of the continuous substance. 

The thermosphere, which gets its name from the heat released in these reactions, is a complex mixture of atoms, ions, and molecules, including a high mole fraction of oxygen atoms. At the same time, however, the total... 

Excited states play important roles in chemistry. Recall from Chapter 7 that the properties of atoms can be studied by observing excited states. In fact, chemists and physicists use the characteristics of excited states extensively to probe the stmcture and reactivity of atoms, ions, and molecules. Excited states also have practical applications. 

Atoms, ions, and molecules rearrange and recombine during chemical reactions. These processes usually do not occur all at once. Instead, each reaction consists of a sequence of molecular events called a reaction mechanism. 

The techniques of X-ray diffraction analyses of crystals of compounds of interest can be used to determine, with high precision, the three-dimensional arrangement of atoms, ions and molecules in such crystals (14) in each case the result is referred to as the "crystal structure." X-ray diffraction by crystals was discovered by von Laue, Friedrich and Knipping (15) and the technique was applied by the Braggs to the determination of the structures of... 

Atoms, ions and molecules present in the stars provide additional opacity at wavelengths corresponding to specific atomic transitions these give rise to comparatively narrow absorption lines (see Fig. 3.2) with intensities related to the abundances of the relevant elements (and much else). Despite the name, processes other than pure absorption (e.g. scattering and fluorescence) are involved in the production of these lines and, while they are often treated in LTE, this is now only a simplifying approximation which often works fairly well, but needs to be checked by more detailed calculations for each particular case. (In some cases, there are even emission lines or emission components, e.g. the solar Ca+ H and K lines in the near UV, which are so strong that the chromosphere affects their central parts.)... 

Gonzalez et al. 2008). Laser ablation is a direct sampling technique by which a high energy laser is focused on the surface of a material and atoms, ions, and particles are ejected. The particles, which are chemically representative of the bulk sample, are then transported into an ICPMS for analysis. In LIBS, a luminous, short-lived plasma is created on the sample surface by the focused laser beam and its emission spectra are analyzed to provide both qualitative and quantitative chemical compositional analysis (Cremers... 

SIMS is by far the most sensitive surface technique, but also the most difficult one to quantify. SIMS is very popular in materials research for making concentration depth profiles and chemical maps of the surface. The principle of SIMS is conceptually simple A primary ion beam (Ar+, 0.5-5 keV) is used to sputter atoms, ions and molecular fragments from the surface which are consequently analyzed with a mass spectrometer. It is as if one scratches some material from the surface and puts it in a mass spectrometer to see what elements are present. However, the theory behind SIMS is far from simple. In particular the formation of ions upon sputtering in or near the surface is hardly understood. The interested reader will find a wealth of information on SIMS in the books by Benninghoven et al. [2J and Vickerman el al. [4], while many applications have been described by Briggs et al. [5]. 

In the literature these studies are classified as imaging mass spectrometry (IMS) and defined as the investigation of the chemical profile of a sample surface with a submicron lateral resolution and chemical specificity. The main aim is to use the power of mass spectrometry techniques to create chemical images showing the distribution of compounds ranging in size from atomic ions and small molecules to large proteins. 

There are two themes in this work (1) that all soil is complex and (2) that all soil contains water. The complexity of soil cannot be overemphasized. It contains inorganic and organic atoms, ions, and molecules in the solid, liquid, and gaseous phases. All these phases are both in quasi equilibrium with each other and are constantly changing. This means that the analysis of soil is subject to complex interferences that are not commonly encountered in standard analytical problems. The overlap of emission or absorption bands in spectroscopic analysis is but one example of the types of interferences likely to be encountered. 

The opportunities for concentrating and detecting (probably primordial) quarks and the properties of adducts of atoms, ions and molecules with quarks are discussed. There is a pronounced difference between positive quarks located in the outer valence-regions (or in the conduction electrons of metals) and negative quarks so firmly bound to nuclei that they may not be mobile, and constitute a kind of new elements with (Z - 1/3). Analogies are drawn with neutrinos, muons and other well-established particles. 

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