Outer shell electronic transitions

The different types of outer shell electronic transitions that occur by absorption or emission of electromagnetic radiation in the UV, Visible and Near IR spectral regions are summarized in Figure 2.1. Since the electronic spectra can be plotted as a function of wavelength (nm), wavenumber (cm" ) or energy (eV), the conversion factors between the various units are reported in Table 2.1. 

Figure Bl.24.14. A schematic diagram of x-ray generation by energetic particle excitation, (a) A beam of energetic ions is used to eject inner-shell electrons from atoms in a sample, (b) These vacancies are filled by outer-shell electrons and the electrons make a transition in energy in moving from one level to another this energy is released in the fomi of characteristic x-rays, the energy of which identifies that particular atom. The x-rays that are emitted from the sample are measured witli an energy dispersive detector.

The question now is, what role do the K, L, M,. . . electrons play in generating the K, L, M,. . . series The answer is not obviously predictable from a knowledge of visible or ultraviolet spectra. Neither hydrogen nor helium has a K series, although each has K electrons. Why Because the K series is generated only when the K shell contains a hole that is filled by an electron that leaves one of the outer (L, M,. . . ) shells or the generation of the K series requires (1) the absence of a K electron, (2) the presence of an outer-shell electron whose transition to the K shell is permitted by the selection rules. This picture explains why—no matter what the method of excitation—all K lines have the same excitation threshold so that all K lines appear together if they appear at all. 

Square-planar stereochemistry is mostly confined to the d8 transition metal ions. The most investigated solvent exchange reactions are those on Pd2+ and Pt2+ metal centers and the mechanistic picture is well established (Table XIV (194-203)). The vast majority of solvent exchange reactions on square-planar complexes undergo an a-activated mechanism. This is most probably a consequence of the coordinatively unsaturated four-coordinate 16 outer-shell electron complex achieving noble gas... 

In Auger transitions, incident electrons interact with the inner shell electrons E of the sample. The vacancy created by an ejected inner shell electron is filled by an outer shell electron (Ei), and a second outer shell electron ( 2) is ejected leaving the atom in a doubly ionized state. The electrons ejected from the outer shells are called Auger electrons, named after the Frenchman Pierre Auger, who discovered the effect. Thus, AES measures the energies of the Auger electrons ( a) emitted from the first 10 A of a sample surface. The energy equation is expressed as... 

Figure 28-3 Schematic representation of the molecular orbitals and bonding electrons of methanal and the n — tt and tt — tt transitions. Only the outer-shell electrons are shown for carbon and oxygen.

Ferrocene is only one of a large number of compounds of transition metals with the cyclopentadienyl anion. Other metals that form sandwich-type structures similar to ferrocene include nickel, titanium, cobalt, ruthenium, zirconium, and osmium. The stability of metallocenes varies greatly with the metal and its oxidation state ferrocene, ruthenocene, and osmocene are particularly stable because in each the metal achieves the electronic configuration of an inert gas. Almost the ultimate in resistance to oxidative attack is reached in (C5H5)2Co , cobalticinium ion, which can be recovered from boiling aqua regia (a mixture of concentrated nitric and hydrochloric acids named for its ability to dissolve platinum and gold). In cobalticinium ion, the metal has the 18 outer-shell electrons characteristic of krypton. 

Coordinatively saturated — A transition metal - complex that has formally 18 outer shell electrons at the central metal atom. 

Answer 9.3 (a) The maximum oxidation state observed for Ac-Np corresponds to the number of outer-shell electrons. Similar behaviour is seen in transition-metal chemistry, for example for the metals Sc-Mn in the 3d series. 

The pattern of oxidation state here is very different to that of the lanthanides, where the common oxides are Ln203. The early actinides in particular show much more resemblance to the transition metals, where the maximum oxidation state corresponds to the number of outer shell electrons this reflects the greater availability of d and f electrons in the actinide elements. The +4 state in particular is more stable than in the lanthanide series, where it is only encountered in a few ions. 

An isolated atom has a characteristic set of discrete energy levels. Then if one electron is removed from an inner shell of the atom, electronic relaxation occurs due to one of the outer shell electrons filling the electron hole left in the inner shell. This leads to the emission of a characteristic X-ray. In such cases, only one spectral line, originating from the X-ray transition between an inner shell and one of the discrete outer shells, can be observed. However, when multiple ionization occurs during a single excitation process, as in the case of enei etic ion impact, a fine structure or finger pattern is necessarily observed in the spectrum. In PIXE,... 

A number of characteristic X-rays can be emitted from an atom if a number of inner shell electrons have been knocked out by high energy particles. The individual characteristic X-rays are marked Ka, Kf5... as mentioned in Chapter 2. It seems that there are many possible ways in which outer shell electrons can fill inner shell vacancies however, the possibilities are limited and such electron transitions in an atom are controlled by the selection rules. 

Quantum theory states that orbiting electrons of an atom must occupy discrete energy levels in order to be stable. Bombardment with ions of sufficient energy (usually megaelectronvolt protons) produced by an ion accelerator will cause inner-shell ionisation of atoms in a specimen. Outer-shell electrons drop down to replace inner shell vacancies, however only certain transitions are allowed. X-rays of a characteristic energy of the element are emitted. An energy dispersive detector is used to record and measure these X-rays. Only elements heavier than fluorine can be detected. 

TT (double or triple bond) and n (outer-shell) electrons are responsible for most UV and visible electron transitions. 

There is an older, alternate naming system for the transitional metals when they ionically bond to form salt compounds. There may be chemicals encountered still using this older naming system, therefore, responders should be familiar with it. In this system, the suffixes ic and ous are used to indicate the higher and lower valence numbers (outer-shell electrons) of a transitional metal. For example, if copper I combines with chlorine, the name would be cuprous chloride. If copper I combines with oxygen, the name would be cuprous oxide. The lowest number of electrons in the outer shell of copper is one. When the metal with the lowest number of electrons is used, the suffix in the alternate naming system is ous. When the metal with the highest number of electrons is used, the suffix is ic. If copper n conbined with phosphorus, it would create cupric phosphide. For example, copper II combined with chlorine would create cupric chloride. 

The charge can be found as the group number on the Periodic Table at the top of the appropriate column. This group number is also the number of outer-shell electrons. The transition metal charge can then be determined by reversing the subscript numbers in the formula to the top of the elements. For example ... 

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