THE SECOND PERIOD, ELEMENTS

In this section, elements 4-10 will be discussed and placed in file periodic table to complete a period in the table. 

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Among the diatomic molecules of the second period elements are three familiar ones, N2,02, and F2. The molecules Li2, B2, and C2 are less common but have been observed and studied in the gas phase. In contrast, the molecules Be2 and Ne2 are either highly unstable or nonexistent. Let us see what molecular orbital theory predicts about the structure and stability of these molecules. We start by considering how the atomic orbitals containing the valence electrons (2s and 2p) are used to form molecular orbitals. 

The relative energies of the molecular orbitals available for occupancy by the valence electrons of the second period elements are shown in Figure 3. This order applies at least through N2- ... 

In general, first bond the multicovalent atoms to each other and then, to achieve their normal covalences, bond them to the univalent atoms (H, Cl, Br, I, and F). If the number of univalent atoms is insufficient for this purpose, use multiple bonds or form rings. In their bonded state, the second-period elements (C, N, O, and F) should have eight (an octet) electrons but not more. Furthermore, the number of electrons shown in the Lewis structure should equal the sum of all the valence electrons of the individual atoms in the molecule. Each bond represents a shared pair of electrons. 

Changes in the radii of atoms of the second period elements according to their atomic numbers. 

In an electron dot diagram, the symbol of the element represents the nucleus of the atom plus its inner shells of electrons, and dots around the symbol stand for the valence electrons. The dots are placed arbitrarily to the left or right or above or below the symbol. In unbonded atoms, two dots, at most, are located in each position. For example, atoms of the second period elements may be represented as follows ... 

The horizontal rows of elements in the periodic table are called periods. Horizontal row one is called the first period (it contains H and He) row two is called the second period (elements Li through Ne) and so on. 

Both the b and k values were estimated by regression analysis from the Taft steric constant Es and the Charton steric constant in an iterative procedure using 96 substituents. The k values are equal to 1.0 for the second period elements, except for the fluorine atom (k = 0.8) k = 1.2,1.3, and 1.7 for the third, fourth, and fifth period elements, respectively. 

Some of the second-period elements show a similarity to the element one column to the right and one row down. For instance, Li is similar in many respects to Mg, and Be is similar to Al. This has been attributed to the charge density on the stable ions (Li vs. Mg Be vs. Al ). From the values of electronic charge (Chapter 5) and ionic radii (Chapter 6), calculate the charge density for these four ions, in coulombs per cubic angstrom. 

Which supposed homonuclear diatomic molecules of the second-period elements should have zero bond order ... 

The process of the completion of the electronic shell is repeated as lor the second period elements and is shown by the electronic formulae ... 

Write formulas for and name the binary hydrogen compounds of the second-period elements (Li to F). Describe how the physical and chemical properties of these compounds change from left to right across the period. 

Write the formulas and names of the oxides of the second-period elements (Li to N). Identify the oxides as acidic, basic, or amphoteric. 

As mentioned earlier, the octet rule applies mainly to the second-period elements. Exceptions to the octet rule fall into three categories characterized by an incomplete octet, an odd number of electrons, or more than eight valence electrons around the central atom. 

Atoms of the second-period elements cannot have more than eight valence electrons around the central atom, bnt atoms of elements in and beyond the third period of the periodic table form some compounds in which more than eight electrons snrronnd the central atom. In addition to the 3s and 3p orbitals, elements in the third period also have 3d orbitals that can be nsed in bonding. These orbitals allow an atom to form an expanded octet. One componnd in which there is an expanded octet is snlfnr hexafluoride, a very stable compound. The electron configuration of snlfnr is [Ne]3x 3p". In SFg, each of snlfnr s six valence electrons forms a covalent bond with a flnorine atom, so there are twelve electrons around the central sulfur atom ... 

Summarize the essential features of the Lewis octet rule. The octet rule applies mainly to the second-period elements. Explain. 

You may have noticed an interesting connection between hybridization and the octet rule. Regardless of the type of hybridization, an atom starting with one s and three p orbitals would still possess four orbitals, enough to accommodate a total of eight electrons in a compound. For elements in the second period of the periodic table, eight is the maximum number of electrons that an atom of any of these elements can accommodate in the valence shell. It is for this reason that the octet rule is usually obeyed by the second-period elements. 

Later in this section we will study molecules formed by atoms of the second-period elements. Before we do, it will be instructive to predict the relative stabilities of the simple species H2, H2, He, and Hc2, using the energy-level diagrams shown in Figure 10.24. The rri and of orbitals can accommodate a maximum of four electrons. The total number of electrons increases from one for H2 to four for He2. The Pauli exclusion principle stipulates that each molecular orbital can accommodate a maximum of two electrons with opposite spins. We are concerned only with the ground-state electron configurations in these cases. 

TABLE 10.5 Properties of Homonuclear Diatomic Molecules of the Second-period Elements ... 

The valence electrons of elements in the second period are in the n = 2 energy level. (Remember that you must fill the m = 1 level with two electrons before adding electrons to the next level.) The third electron of lithium (Li) and the remaining electrons of the second period elements must be in the n = 2 level and are considered the valence electrons for lithium and the remaining second period elements. 

The atom is composed of the kernel and the outer atom or shell, which in the case of the neutral atom, contains negative electrons in equal numbers to the excess of positive charges of the kernel. Today we refer to the electrons in the outer shell as valence electrons. Lewis suggested the use of the atomic symbol in boldface to represent the kernel of the atom and that valence electrons should be indicated by dots. Figure 7.1 shows the electronic structures of the second-period elements from Li to F as suggested by Lewis. 

Fig. 7.1. Above the electronic structures of the second-period elements from Li to F according to Lewis [1]. Below the electronic structure of the iodine molecule.

Table 9.1. Dissociation energies and bond distances in gaseous homonuclear diatomic species (charged or uncharged) formed from the second period elements from lithium to neon. ...

It is generally true that the properties of chemical compounds formed by the second period elements from Li to He differ from the analogous formed by the heavier elements in the same group. Nowhere, however, is the difference so great as between the compounds formed by carbon on the one hand and those of silicon, gemanium, tin, or lead on the other ... 

As we have seen, the second-period element, fluorine, induces hypervalency in many other p-block elements, but it does not itself display hypervalency. The third and fourth period halogens, i.e. chlorine and bromine, form trifluorides and pentafluorides, XF3 and XF5. The fifth-period element, iodine, forms an unstable trifluoride which decomposes below room temperature, a pentafluoride and a heptafluoride. Iodine also forms a solid trichloride which decomposes on evaporation as in Groups 15 and 16 we find that the greater number of hypervalent derivatives are formed by the fifth-period element. 

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