Main-group elements valence electrons

For the main-group elements, valence electrons (those involved in reactions) are in the outer (highest energy) level only. For transition elements, (n - 1 )d electrons are also considered valence electrons. 

For main group elements the number of framework electrons contributed is equal to (t + a — 2) where v is the number of valence shell electrons of that element, and x is the number of electrons from ligands, eg, for Ff, x = and for Lewis bases, x = 2. Examples of 2n + 2 electron count boranes and heteroboranes, and the number of framework electrons contributed by their skeletal atoms, ate given in Table 1. 

The number of valence electrons in an atom of a main-group element such as nitrogen is equal to its group number. In the case of nitrogen this is five. 

These structures (without the circles) are referred to as Lewis structures. In writing Lewis structures, only the valence electrons written above are shown, because they are the ones that participate in covalent bonding. For the main-group elements, the only ones dealt with here, the number of valence electrons is equal to the last digit of the group number in the periodic table (Table 7.1). Notice that elements in a given main group all have the same number of valence electrons. This explains why such elements behave similarly when they react to form covalently bonded species. 

FIGURE 1.52 The successive ionization energies of a selection of main-group elements. Note the great increase in energy required to remove an electron from an inner shell. In each case, the blue rectangle denotes ionization from the valence shell. 

Formulas of compounds consisting of the monatomic ions of main-group elements can be predicted by assuming that cations have lost all their valence electrons and anions have gained electrons in their valence shells until each ion has an octet of electrons, ora duplet in the case of FI, Li, and Be. 

For low values of the valence electron concentration (VEC< 4 for main group elements), covalent 2c2e bonds are not sufficient to overcome the electron deficiency. We have the case of electron-deficient compounds . For these, relief comes from multicenter bonds. In a three-center two-electron bond (3c2e) three atoms share an electron pair. An even larger number of atoms can share one electron pair. With increasing numbers of... 

The Wade rules can be applied to ligand-free cluster compounds of main-group elements. If we postulate one lone electron pair pointing outwards on each of the n atoms, then g — 2n electrons remain for the polyhedron skeleton (g = total number of valence elec-... 

The elements may be divided into types (Fig. 17-10), according to the position of the last electron added to those present in the preceding element. In the first type, the last electron added enters the valence shell. These elements are called the main group elements. In the second type, the last electron enters a d subshell in the next to last shell. These elements are the transition elements. The third type... 

Tin belongs to the long period elements from Rb to Xe and is a main group element because the 4d shell is filled with electrons. Since the valence electrons are 5s2p2, tin occurs in two valences. Whereas valence 2 is formally always positive, valence 4 has amphoteric properties possessing the formal oxidation states +4 or —4, according to the covalently bound substituents and to the reaction partner. 

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