Main-group elements electron configurations

As we discussed in Chapter 9, valence electrons are those in the outermost principal shell. Since valence electrons are most important in bonding, Lewis theory focuses on these. In Lewis theory, the valence electrons of main-group elements are represented as dots siuToimding the symbol of the element. The result is called a Lewis stmcture, or dot structure. For example, the electron configuration of O is ... 

The fundamental difference between transition metal elements and the Main Group Elements is the configuration of the valence electron shells. Main Group Elements have, if any, only closed d-shells. 

Bond energy variations over the periodic table will be subject to perturbations which reflect the underlying atomic configurations. Compounds derived from main-group elements of Period 4, for example, will show discontinuities in properties from those of Period 3 because of the extra d-electron shell. Conversely, the insertion of an f-electron shell brings together the properties of the second and third transition series, especially in the earlier groups. 

Considering the influence of electronic configurations on crystal structures it may be asked, whether certain structure t5rpes are restricted to fluorine compounds of the transition elements. Apart from the structure types distorted by the Jahn-Teller effect such a limitation is not obvious at all. On the contrary quite a number of structure prototypes are represented by compounds of the main group elements. Bonding thus must be similar in both, main group and transition element fluorides, at least as for the factors that influence crystal structmes. 

Main Group Element elements in groups 1, 2, and 13-18, members in each group have the same general electron configuration Mass Defect the change in mass when a nucleus is formed from its constituent nucleons... 

The s and p blocks form the main groups of the periodic table. The similar electron configurations for the elements in a main group are the reason for the similar properties of these elements. The group number tells us how many valence-shell electrons are present. In the s block, the group number (1 or 2) is the same as the number of valence electrons. In the p block, we have to subtract 10 (for the 10 -electrons) from the group number to find the number of valence electrons. For example, fluorine in Group 17 has seven valence electrons. 

In compounds containing heavy main group elements, electron correlation depends on the particular spin-orbit component. The jj coupled 6p j2 and 6/73/2 orbitals of thallium, for example, exhibit very different radial amplitudes (Figure 13). As a consequence, electron correlation in the p shell, which has been computed at the spin-free level, is not transferable to the spin-orbit coupled case. This feature is named spin-polarization. It is best recovered in spin-orbit Cl procedures where electron correlation and spin-orbit interaction can be treated on the same footing—in principle at least. As illustrated below, complications arise when configuration selection is necessary to reduce the size of the Cl space. The relativistic contraction of the thallium 6s orbital, on the other hand, is mainly covered by scalar relativistic effects. 

The valence ns and np electrons play important roles in the chemistry of main group elements, in contrast to the d electrons in the chemistry of transition metals. In Figure 1 are shown the radii of atomic orbitals (maximal electron-density), which are calculated for group 14 elements. It should be noted that the valence ns and np atomic orbitals show great difference in their sizes for the heavier atoms (Si, Ge, Sn, Pb), though the size of the 2s atomic orbital of carbon is almost equal to that of the 2p atomic orbitals. Therefore, the heavier atoms have a lower tendency to form s-p hybrid orbitals with high p character and they prefer to retain the ns np valence electronic configuration, in contrast to the case of carbon. 

---