Second period elements, 158 table

Table 1-1. Covalences of H and Second-Period Elements in Groups 2 through 7...

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. 

The unique properties of carbon relate to its position in the periodic table. As a second-period element, carbon atoms are relatively small. Therefore, it can easily form the double and triple bonds that are rare in the compounds of related elements, such as silicon. As a Group IV element, carbon can form four bonds, which is more than the other second-period elements this characteristic gives it wide... 

The element carbon has a rich and varied chemistry because of its location in the periodic table. As a Group IV element, each carbon atom forms four covalent bonds, more than any other second-period element. In consequence of its intermediate value of electronegativity, carbon can bond with more electronegative... 

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 ... 

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. 

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

Equation 102. Values of A are given in Table 38. Certain relationships between A and the Pauling electronegativity are observed. Thus, for groups of the type WZ where W is a second period element (0,N,F) and Z is H, Ak, or a lone pair, and for the halogen substituents, the equation... 

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. ...

TABLE 10.7 The experimentally determined energy gap between the 2s and 2p orbitals for the second period elements. 

Period Referring to position in the periodic table (also see row ). In the current study, the second period elements include carbon and oxygen, and the third row elements include silicon and sulfur. Note Period and row are not always synonymous in their numbering. 

As we noted in Section 22.1, a second-period element is often considerably different from the other elements in its column. Those differences become more pronounced as you progress to the right in the periodic table. We briefly noted differences in properties of lithium from those of the other alkali metals, though those differences are not great. Beryllium, however, shows rather marked differences from the other alkaline earth elements. We have already noted that beryUium differs in its lack of reactivity compared with the other Group IIA metals. Another notable diffoence is in the properties of the hydroxides. Whereas those of the elements magnesium to barium are basic, beryllium hydroxide is amphoteric, reacting with both acids and bases. 

As a result, even though the number of interelectronic interaction integrals of various types increases appreciably, the overall number of the parameters for the second-period elements is reduced from 102 to 41. Using this method, the computer calculation takes 1.5 times longer than with the MINDO/3 method. As may be seen from Table 2.4 compiled from the data presented by Dewar [53,62] for over 200 compounds, the MNDO method achieves a better agreement with experiment than the MINDO/3 method. 

Table 1.6. Valence electron configurations of diatomic molecules of second period elements...

Carbon and silicon differ in atomic size (covalent radii rc = 77 pm rj = 117 pm) and electronegativity [1] (xc = 2.50 Xsi = 1-74). The electronegativity of silicon is peculiar. This finding was recently corroborated, and a new electronegativity scale for the group IV elements estimated from the observed bond distances in the bivalent and tetravalent halides. The predicted values are C, 2.6 Si, 1.9 Ge, 2.5 and Sn, 2.3 [2]. Table 1.1 shows that the differences between the covalent radii and the electronegativities of homologoues first and second period elements are similar to those of carbon and silicon. 

Other exceptions to the octet rule include molecules with incomplete octets—usually totaling 6 electrons (especially important in compounds containing boron)—and molecules with expanded octets—usually 10 or 12 electrons (which can occur in compounds containing elements from the third row of the periodic table and below). Expanded octets never occur in second-period elements. 

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