Electronic shell completed

If IS offen convenienf to speak of the valence electrons of an atom These are the outermost electrons the ones most likely to be involved m chemical bonding and reac tions For second row elements these are the 2s and 2p electrons Because four orbitals (2s 2p 2py 2pf) are involved the maximum number of electrons m the valence shell of any second row element is 8 Neon with all its 2s and 2p orbitals doubly occupied has eight valence electrons and completes the second row of the periodic table... 

Atoms and free radicals are highly reactive intermediates in the reaction mechanism and therefore play active roles. They are highly reactive because of their incomplete electron shells and are often able to react with stable molecules at ordinary temperatures. They produce new atoms and radicals that result in other reactions. As a consequence of their high reactivity, atoms and free radicals are present in reaction systems only at very low concentrations. They are often involved in reactions known as chain reactions. The reaction mechanisms involving the conversion of reactants to products can be a sequence of elementary steps. The intermediate steps disappear and only stable product molecules remain once these sequences are completed. These types of reactions are refeiTcd to as open sequence reactions because an active center is not reproduced in any other step of the sequence. There are no closed reaction cycles where a product of one elementary reaction is fed back to react with another species. Reversible reactions of the type A -i- B C -i- D are known as open sequence mechanisms. The chain reactions are classified as a closed sequence in which an active center is reproduced so that a cyclic reaction pattern is set up. In chain reaction mechanisms, one of the reaction intermediates is regenerated during one step of the reaction. This is then fed back to an earlier stage to react with other species so that a closed loop or... 

Figure 10. Electron shell closure fails to coincide with the dosing of periods in the periodic table because the shells do not fill in strictly sequential order As shown here, die fourth shell begins to fill before the third shell has been completed. The resumption of third-shHl filling accounts for the appearance of the first transition-metal series, beginning with scandium and ending with zinc.

But does the fact that the third shell can contain 18 electrons, for example, which emerges from the relationships among the quantum numbers, also explain why some of the periods in the periodic system contain eighteen places Actually not exactly. If electron shells were filled in a strictly sequential manner there would be no problem and the explanation would in fact be complete. But as everyone is aware, the electron shells do not fill in the expected sequential manner. The configuration of element number 18, or argon is,... 

Elements at the right of the p block have characteristically high electron affinities they tend to gain electrons to complete closed shells. Except for the metalloids tellurium and polonium, the members of Groups 16/VI and 17/VII are nonmetals (Fig. 1.62). They typically form molecular compounds with one another. They react with metals to form the anions in ionic compounds, and hence many of the minerals that surround us, such as limestone and granite, contain anions formed from non-metals, such as S2-, CO,2-, and S042-. Much of the metals industry is concerned with the problem of extracting metals from their combinations with nonmetals. 

All elements in the s block are reactive metals that form basic oxides. The p-block elements tend to gain electrons to complete dosed shells they range from metals through metalloids to nonmetals. 

Studies of the electron distributions around outer atoms consistently show that hydrogen is always associated with two electrons (one pair). All other outer atoms always have eight electrons (four pairs). The reason for this regularity is that each atom in a molecule is most stable when its valence shell of electrons is complete. For hydrogen, this requires a single pair of electrons, enough to make full use of the hydrogen 1 S orbital. Any other atom needs four pairs of electrons, the maximum number that can be accommodated by an .S p valence shell. Details of these features can be traced to the properties of atoms (Chapter 8) and are discussed further in Chapter 10. 

To reach the lower energy state of a filled energy shell, atoms sometimes share more than one electron. Oxygen, for example, has an outer p orbital with six electrons. The most common form of oxygen is O2. To complete the electron shells of both atoms, they must share two electrons. The reaction to form the molecule and its structure would then be represented as ... 

Since the possible numerical values of / depend on the value of n, the number of subshells within a given shell is determined by the value of n. The number of subshells within a given shell is merely the value of rt, the shell number. Thus, the first shell has one subshell the second shell has two subshclls, and so forth. These facts are summarized in Table 17-4. Even the atoms with the most electrons do not have enough electrons to completely fill the highest shells shown. The subshells that hold electrons in the ground states of the biggest atoms are boldfaced. 

Using electron dot notation, the production of sodium fluoride, magnesium fluoride, and magnesium oxide may be pictured as follows A sodium atom and a fluorine atom react in a 1 1 ratio, since sodium has one electron to lose from its outermost shell and fluorine requires one more electron to complete its outermost shell. 

To lose its entire outermost shell, a magnesium atom must lose two electrons. Since each fluorine atom needs only one electron to complete its octet, it takes two fluorine atoms to react with one magnesium atom ... 

What was the importance of this research result for the chirality problem One difficulty is provided by the fact that the interaction responsible for the violation of parity is in fact not so weak at all, although it only acts across a very short distance (smaller than an atomic radius). Thus, the weak interaction is not noticeable outside the atomic nucleus, except for p-decay. It would thus have either no influence on chemical reactions or only a very limited effect on chemical reactions, as these almost completely involve only interactions between the electron shells. 

All the halogens form salts of this kind, usually with elements from Groups I and II. They combine easily with the alkali metals because the alkali metals have electrons which are easy to lose, and the halogens need these extra electrons to complete their outer shells. 

If you move left one column in the periodic table from the halides, the chalcogenides need two electrons to complete their valence shell, and thus can bond to the surface and each other simultaneously. This appears to account for much of the interesting surface chemistry of chalcogenide atomic layers. Chalcogenides, including oxides (corrosion), are some of the most studied systems in surface chemistry. The oxides are clearly the most important, but significant amounts of work have been done with sulfur, selenium and tellurium. 

It is seen from their orbital structures that hydrogen and fluorine both need to share 1 electron to complete their outer shells. Therefore the orbital representation of HF molecule is ... 

According to the conventional simple chemical definition, the valence compounds are those in which individual atoms are assumed to reach a filled valence shell by accepting, donating or sharing electrons. This definition was first applied to the compounds (normal valence compounds) where the cations may donate the exact number of electrons to complete the valence shells (particularly the octet shells) of every anion. 

Fluorine does not occur in a free state in nature, and because fluorine is one of the most reactive elements, no chemical can free it from any of its many compounds. The reason for this is that fluorine atoms are the smallest of the halogens, meaning the electron donated by a metal (or some nonmetals) are closer to fluorines nucleus and thus exert a great force between the fluorine nuclei and the elements giving up one electron. The positive nuclei of fluorine have a strong tendency to gain electrons to complete the outer shell, which makes it a strong oxidizer. 

Helium (H ) is not really one of the noble gases, but because it is inactive and has a completed first shell (K = 2 electrons), it is placed at the top of group 18 (VIIIA). Radon, which has the largest molecular size of the noble gases, is the only radioactive noble gas. Its outer electron shell is furthest from the nucleus. 

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