Outer-level electrons

The second period begins with lithium, which has three electrons—two in energy level one and one in energy level two. Lithium has one electron in its outer energy level. To the right of lithium is beryllium with two outer-level electrons, boron with three, and so on until you reach neon with eight. 

Why do you need to know how to determine the munber of outer-level electrons that are in an atom Recall that at the beginning of this section, it was stated that when atoms come near each other, it is the electrons that interact. In fact, it is the valence electrons that interact. Therefore, many of the chemical and physical properties of an element are directly related to the munber and arrangement of valence electrons. 

Element X is in the fourth period. Its outer energy level has three electrons. How does the number of outer-level electrons of element X compare with that of element Y, which is in the sixth period. Group 13 Write the name and symbol of each element. 

Atoms become stable by reacting to achieve the outer-level electron structure of a noble gas (Group 18). 

When sulfur reacts with metals, it often forms an ionic compound. Draw a Lewis dot structure of a sulfur atom. Then, draw the Lewis structure of the ion it will form. Name an element that has the same outer-level electron structure as a sulfur ion. 

An outer-level electron pair that is not involved in bonding is called a lone pair, or nnshared pair. The bonding pair in HF fills the outer level of the H atom and, together with three lone pairs, fills the outer level of the F atom as well ... 

The values decrease down each group, because the increasing atomic radius means a looser hold on outer level electrons and the effect of the increasing nuclear positive charge is weakened by the screening effea of the extra inner shells. 

When a chemical element is bombarded by high-energy particles, orbital electrons may be ejected creating inner orbital atomic vacancies. These vacancies may be filled by transition of outer level electrons giving rise to characteristic X-radiation. X-ray fluorescence spectrometry provides the means of identification of an element by measurement of its characteristic X-ray emission wavelength of energy. 

With AES, the sample is subjected to a high-energy (typically 2-20 KeV) electron beam that can cause ejection of a core electron from an atom to form an atomic inner shell vacancy. An outer-level electron will then fill the inner-level vacancy, which will induce an excited state. One of the ways that the atom can then relax is by emitting another electron to form a doubly ionized species. This electron is the Auger electron (named for Pierre Auger, who recognized the effect... 

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 concept of oxidation states is best applied only to germanium, tin and lead, for the chemistry of carbon and silicon is almost wholly defined in terms of covalency with the carbon and silicon atoms sharing all their four outer quantum level electrons. These are often tetrahedrally arranged around the central atom. There are compounds of carbon in which the valency appears to be less than... 

The concept has been generalized in the ONIOM method to include several layers, for example using high level ab initio (e.g. CCSD(T)) in the central part, lower-level electronic structure theory (e.g. MP2) in an intermediate layer and a force field to treat the outer layer. 

It is possible to explain these trends in terms of the electron configurations of the corresponding atoms. Consider first the increase in radius observed as we move down the table, let us say among the alkali metals (Group 1). All these elements have a single s electron outside a filled level or filled p sublevel. Electrons in these inner levels are much closer to the nucleus than the outer s electron and hence effectively shield it from the positive charge of the nucleus. To a first approximation, each inner electron cancels the charge of one pro-... 

Let us start at an elementary level or with a typically "chemical" view. Suppose we ask an undergraduate chemistry student how quantum mechanics explains the periodic table. If the student has been going to classes and reading her book she will respond that the number of outer-shell electrons determines, broadly speaking, which elements share a common group in the periodic table. The student might possibly also add that the number of outer-shell electrons causes elements to behave in a particular manner. 

Section 18.2). The latest generation of such catalysts (1 in Fig. 18.17) reproduces the key features of the site (i) the proximal imidazole ligation of the heme (ii) the trisi-midazole ligation of distal Cu (iii) the Fe-Cu separation and (iv) the distal phenol covalently attached to one of the imidazoles. As a result, binding of O2 to compound 1 in its reduced (Fe Cu ) state appears to result in rapid reduction of O2 to the level of oxides (—2 oxidation state) without the need for outer-sphere electron transfer steps [Collman et ah, 2007b]. This reactivity is analogous to that of the heme/Cu site of cytochrome c oxidase (see Section 18.2). 

In the case of boron impurities a complementary situation occurs. Boron has only three outer bonding electrons instead of the four found on carbon. Each boron impurity atom occupies a carbon position, forming Be, which results in the creation of a set of new acceptor energy levels just 0.64 x 10 19 J (0.4 eV) above the valence band. The transition of an electron from the valence band to this acceptor level has an absorption peak in the infrared, but the high-energy tail of the absorption band spills into the red at 700 nm. The boron-doped diamonds therefore absorb some red light and leave the gemstone with an overall blue color. 

These electronic interpretations of valency allow us to interpret the phenomenon of variable valency exhibited by many of the transition metal elements. As shown in Fig. 10.5 (Chapter 10), the transition metals exist because the energy of the outer d orbitals lies between the 5 and p energy levels of the next lowest orbitals, and thus are filled up in preference to the p orbitals. Copper, for example (1 s22s22p63s23p63dl04sl), has a single outer s electron available for bonding, giving rise to Cu(I) compounds, but it can also lose one of the 3d electrons, giving rise to Cu(II) compounds. 

We examine an electron transfer of hydrated redox particles (outer-sphere electron transfer) on metal electrodes covered with a thick film, as shown in Fig. 8-41, with an electron-depleted space charge layer on the film side of the film/solution interface and an ohmic contact at the metal/film interface. It appears that no electron transfer may take place at electron levels in the band gap of the film, since the film is sufficiently thick. Instead, electron transfer takes place at electron levels in the conduction and valence bands of the film. 

In alkali metal intercalation compounds, the guest is ionised in the host, donating its outer s electron to the host s electronic energy levels. Thus there are two aspects to consider, the sites where the ion resides, and the energy levels or bands that the electron occupies. Guests such as water that remain neutral will only be discussed in the section on cointercalation. In some hosts, notably graphite, some guests accept electrons from the... 

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