The Third Period

Referring to the periodic table as needed write electron config urations for all the elements in the third period... 

The third period begins with sodium and ends with argon The atomic number Z of sodium is 11 and so a sodium atom has 11 electrons The maximum number of electrons in the Is 2s and 2p orbitals is ten and so the eleventh electron of sodium occupies a 3s orbital The electron configuration of sodium IS 2s 2p 2p 2p is ... 

The elements in Groups 1 and 2aref3Bng an ssubkveL Thus Li and Be in the second period fill the 2s subleveL Na and Mg in the third period fill the 3s sublevel, and so on. 

The elements in Groups 13 through 18 (six dements in each period) fill p sublevels, which have a capacity of six electrons. In the second period, the 2p sublevel starts to fill with B (Z = 5) and is completed with Ne (Z = 10). In the third period, the elements A1 (Z = 13) through Ar (Z = 18) fill the 3p subleveL... 

The decrease in atomic radius moving across the periodic table can be explained in a similar manner. Consider, for example, the third period, where electrons are being added to the third principal energy level. The added electrons should be relatively poor shields for each other because they are all at about the same distance from the nucleus. Only the ten core electrons in inner, filled levels (n = 1, n = 2) are expected to shield the outer electrons from the nucleus. This means that the charge felt by an outer electron, called the effective nuclear charge, should increase steadily with atomic number as we move across the period. As effective nuclear charge increases, the outermost electrons are pulled in more tightly, and atomic radius decreases. 

The third period is characterized by the extensive studies, both in the USSR and abroad, of the structure, properties, and bond characteristics of peroxide compounds. This period includes the work of Kazamovskii and his coworkers concerning the structure of a series of peroxide compounds, his discovery of sodium superoxide, and the fundamental investigations carried out by the Canadian scientist Otto Maas and his co-workers concerning concentrated hydrogen peroxide. . . ... 

The third period corresponds to the last five data points where it is obvious that the assumption of a nearly constant qM is not valid as the slope changes essentially from point to point. Such a segment can still be used and the integral method will provide an average qM, however, it would not be representative of the behavior of the culture during this time interval. It would simply be a mathematical average of a time varying quantity. 

Electronegativity arguments obviously cannot explain the lack of trend in chemical shift for the third-period binary fluorides. The unexpectedly large shielding exhibited for SF2 and C1F has been attributed toai -> a excitation caused by the external magnetic field.1... 

Starting at the first electron added to an atom, (a) what is the number of the first electron in the second shell of the atom (b) What is the atomic number of the first element of the second period (c) What is the number of the first electron in the third shell of the atom (d) What is the atomic number of the first element of the third period (e) What is the number of the first electron in the fourth shell of the atom (/) What is the atomic number of the first element of the fourth period ... 

The second approach typically involves expanding the wave functions in terms of atomic or atomic-like orbitals. Frequently s- and p-symmetry functions suffice for silicon and impurities up through the third period. A minimal basis set for silicon would consist of four basis functions, one 5-function and three p-functions on each atom. Some approaches supplement the minimal basis either with more atomic-like functions or with additional types of functions, such as plane waves. Some calculations use only plane waves for the basis. 

Example The third period of the periodic table (sodium to argon)... 

With Na, the electron configuration of which may also be described as [Ne s1, the third period begins. A similar situation is found for each of the other periods in the Table the number of the period is the principal quantum number of the least tightly bound electron of the first element (an alkali metal) of the period. A few more details of these questions and the characteristics of special points in the Periodic Table are discussed in following paragraphs. The electron configurations of all the elements are given in Chapter 5. 

Although the author participated in all three periods, this historical perspective covers only the first two periods. The perspective on the important developments in the third period as described above can be found throughout the book (especially in Chapters 6 and 7). 

Two comparisons are discussed in this section (a) the acid strengths of the oxoacids of the second period, H3B03, H2C03 and HN03, and (b) those of the oxoacids of the third period, H3P04, H2S04 and HC104. 

The M shell begins to develop with the elements which follow neon, and after eight places in the table, argon occurs with eight electrons in the M shell, and is, like neon, a noble gas. Argon is the last element in the third period, and although the M shell is not yet complete, the next electron shell begins to develop in the elements... 

It is due to the decrease of the heat of formation that, at the end of the periods, the highest halides, oxides, sulphides and nitrides become unstable. In the second period no normal halides are formed after carbon. In the third period the fluorides extend to SFe, in the fifth group to IF7. The normal oxides, too, extend to higher valencies in the third period, where are S03 and C1207, than in the second, where N2Os is the highest oxide. 

From the point of view of practical chemistry, an important group of complexes are those obtained by the reaction of water with oxides. Starting with the oxides of the third period, in accordance with Section 29, the following complex ions can be expected to be formed... 

The s- and p-orbitals of the shell with n = 3 are full by the time we get to argon, [Ne]3s23p6, which is a colorless, odorless, unreactive gas resembling neon. Argon completes the third period. In accord with Figs. 

Expansion of Octet. The octet rule strictly applies to those elements that have only four orbitals available. In those cases, a maximum of four bonds can be formed through overlap. There are, however, a number of molecules where five or six covalent bonds are formed to a central atom. Such behavior is exhibited by elements in the third period and subsequent periods of the periodic table, but never by elements in the second row of the periodic table. A few molecules of this type are listed below. 

FIGURE 14.7 Melting points (in kelvins) of oxides of the third-period elements in their highest oxidation states. (No melting point is given for Na20 because it sublimes and has a vapor pressure of 1 atm at 1548 K.)... 

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