M-shell electrons

The layout of the periodic table (Fig. 2.5) reflects the shell structure of the electrons. Hydrogen and helium have only -shell electrons. The elements in row two have and L-shell electrons, with the Is orbitals always filled and the 2s and 2p orbitals filled in succession. Those in row three have and L-shell electrons, with Is, 2s, and 2p orbitals filled, and the 3 s and 3p orbitals are filled in succession. Elements in the fourth row have K, L, and M-shell electrons, with the Is, 2s, 3s, 2p, and 3p orbitals completely filled. After the 4s orbitals are filled, the 3d orbitals are filled, giving the transition metals. Then come the 4p orbitals. Row five is filled in an analogous fashion. In row six, the lanthanides, which fit between lanthanum and hafnium, reflect the appearance of the N-shell electrons, which fill the f orbitals. Row seven, which contains the actinides, also has K, L, M, and N-shell electrons. 

The nomenclature for X-ray emission consists of the name of the shell in which the vacancy was created (K, L, M, N), and on the electronic shell that filled the vacancy. For instance, ejection of a K shell electron, filled with a L shell electron is denoted as K if filled with an M shell electron, then Kp is used, and so on. Due to electronic subshells, nomenclature becomes significantly complex, as shown in Figure 7.19. 

An LMM Auger electron is one emitted by the following mechanism A vacancy, initially created in the L shell by the photoejection of an electron, is filled by the fall of an M-shell electron, accompanied by the ejection of another M-shell electron from the atom. The second M-shell electron is the LMM Auger electron. 

Figure 1 Energy diagrams for a Kp transition and Kp " satellite transition. A() denotes the state of an atom. For example, A(M L ) denotes an atomic level where an M shell electron has been removed and an L shell electron is in an outer bound (excited) state. The energy difference Ekl-Ek is more than Eml-Em as more energy is required to remove an L shell electron in the presence of a K shell vacancy than in the presence of a M shell vacancy. Solid downward arrows show vacancy transitions, and dashed lines with double arrows show radiative transitions.

Figure 6 Energy diagram for auto-ionization. An L shell electron is excited to an outer nl shell. Subsequently, an M shell electron fills up the L shell vacancy. In case A, the excited electron in the nl shell leaves the atom, and in case B, another M shell electron leaves the atom while the nl shell electron remains a spectator electron.

The covalent, or shared electron pair, model of chemical bonding was first suggested by G N Lewis of the University of California m 1916 Lewis proposed that a sharing of two electrons by two hydrogen atoms permits each one to have a stable closed shell electron configuration analogous to helium... 

In atoms in which electrons in M or A shells take part to some extent in molecular orbital formation some transitions in the L spectmm may be broadened. Similarly, in an M emission spectmm, in which the initial vacancy has been created in the M shell, there is a greater tendency towards broadening due to molecular orbital involvement. 

The arrangement of electrons in an atom is described by means of four quantum numbers which determine the spatial distribution, energy, and other properties, see Appendix 1 (p. 1285). The principal quantum number n defines the general energy level or shell to which the electron belongs. Electrons with n = 1.2, 3, 4., are sometimes referred to as K, L, M, N,. .., electrons. The orbital quantum number / defines both the shape of the electron charge distribution and its orbital angular... 

Hullen-elektron, n. sheath electron, shell electron. -stoff, m. cover material specif., balloon fabric. 

The major features of molecular geometry can be predicted on the basis of a quite simple principle—electron-pair repulsion. This principle is the essence of the valence-shell electron-pair repulsion (VSEPR) model, first suggested by N. V. Sidgwick and H. M. Powell in 1940. It was developed and expanded later by R. J. Gillespie and R. S. Nyholm. According to the VSEPR model, the valence electron pairs surrounding an atom repel one another. Consequently, the orbitals containing those electron pairs are oriented to be as far apart as possible. 

Every chemist knows that atomic nuclei are surrounded by shells of electrons, which, when completed, contain 2 (inner shell), 8, 8, 18,. . . electrons, this being the explanation of periodicity in chemical properties. For reasons that will appear, these may be called (in the same order) K, L, M, N,. . . electrons on the basis of conclusive x-ray evidence. 

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. 

It has been found that no significant error is introduced by combining the subgroups of an entire shell, using an average value of y for the entire K, L, M,. .. shell. With the Stoner distribution of electrons among the levels, this average value is... 

EXAMPLE 17.1. What is the maximum number of electrons that can occupy the M shell ... 

The M shell in an atom corresponds to the third energy level (n =3) hence the maximum number of electrons it can hold is... 

Electrons having the same value of n in an atom are said to be in the same shell. Electrons having the same value of n and the same value of / in an atom are said to be in the same subshell. (Electrons having the same values of n, /, and m in an atom are said to be in the same orbital.) Thus, the first two electrons of magnesium (Table 17-3) are in the first shell and also in the same subshell. The third and fourth electrons are in the same shell and subshell with each other. They are also in the same shell with the next six electrons (all have n = 2) but a different subshell (/ = 0 rather than 1). With the letter designations of Sec. 17.3, the first two electrons of magnesium are in the Is subshell, the next two electrons arc in the 2s subshell, and the next six electrons are in the 2p subshell. The last two electrons occupy the 3s subshell. 

Background alkali metal chemistry. The alkali metals have the lowest ionization potentials of any group in the periodic table and hence their chemistry is dominated by the M+ oxidation state. However, it has been known for some time that a solution of an alkali metal (except lithium) in an amine or ether forms not only M+ ions and solvated electrons but also alkali anions of type M (Matalon, Golden Ottolenghi, 1969 Lok, Tehan Dye, 1972). That is, although an alkali metal atom very readily loses its single s-shell electron ... 

The nomenclature for the emitted x-rays is related to the shells involved in the transition, as illustrated in Fig. 10b. For example, if the initial electron is ejected from the K shell and the second electron drops from the adjacent L shell, a K-a x-ray is emitted. If the electron drops from the M shell, the emitted x-ray is a K-fi x-ray. The most common lines observed are the KIM lines. 

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