Second-row elements

This section started with the discovery of Soddy and Fajans on radioactive decay around 1910 and the relationship of radioactive decay to the periodic table. At this point in the history, we understand the periodic table and we understand the role of isotopes in the periodic table. We have not yet understood the structure of the modern Table, i.e. first row two elements, second row eight elements, etc. That understanding can be based on Bohr theory of the hydrogen atom originally developed in 1911 and is summarized in Bohr s famous article in Zeitschrift fur Physik (Bohr 1922). 

For each element second row gives electron configuration, third gives elechonegativity and important oxidation states. 

First-row elements Second-row elements Third-row elements Fourth-row elements... 

In contrast with first-row main-group elements, second-row and heavier atoms such as P and S can extend their octet, so substitution at these atoms can occur either by an Sn2 mechanism or by a two-step addition-elimination mechanism. In the first step, the nucleophile adds to the electrophilic heavy atom to give a hypervalent, 10-electron in-... 

The best nucleophiles for the SrnI mechanism can make a relatively stable radical in the initiation part, either by resonance (enolates) or by placing the radical on a heavy element (second-row main-group or heavier nucleophiles). The best electrophiles are aryl bromides and iodides. If light is required for substitution to occur, the mechanism is almost certainly SrnI. Substitution at alkenyl C(sp2)-X bonds can also occur by an SrnI mechanism. 

The structure of NaHSOj, sodium bisulfite, is rather curious. It is an oxyanion of a sulfur(IV) compound with a lone pair of electrons—the HOMO— on the sulfur atom, but the charge is formally on the more electronegative oxygen. As a second-row element (second row of the periodic table, that is) sulfur can have more than just eight electrons—it s all right to have four, five, or six bonds to S or P, unlike, say, B or 0. Second-row elements have d orbitals as well as s and p so they can accommodate more electrons. 

The period (or row) of the periodic table m which an element appears corresponds to the principal quantum number of the highest numbered occupied orbital (n = 1 m the case of hydrogen and helium) Hydrogen and helium are first row elements lithium in = 2) IS a second row element... 

If IS offen convenienf to speak of the valence electrons of an atom These are the outermost electrons the ones most likely to be involved m chemical bonding and reac tions For second row elements these are the 2s and 2p electrons Because four orbitals (2s 2p 2py 2pf) are involved the maximum number of electrons m the valence shell of any second row element is 8 Neon with all its 2s and 2p orbitals doubly occupied has eight valence electrons and completes the second row of the periodic table... 

Lewis structures in which second row elements own or share more than eight valence electrons are especially unstable and make no contribution to the true structure (The octet rule may be ex ceeded for elements beyond the second row)... 

Electronegativity The effect of electronegativity on acidity is evident m the following senes involving bonds between hydrogen and the second row elements C N O and F... 

Section 1 3 The most common kind of bonding involving carbon is covalent bond ing A covalent bond is the sharing of a pair of electrons between two atoms Lewis structures are written on the basis of the octet rule, which limits second row elements to no more than eight electrons m their valence shells In most of its compounds carbon has four bonds... 

Valence electrons (Section 1 1) The outermost electrons of an atom For second row elements these are the 2s and 2p elec trons... 

Gordon, M.S. Binkley, J.S. Pople, J.A. Pietro, W.J. Hehre, W.J. Self-consistent molecular-orbital methods. 22. Small split-valence basis sets for second-row elements J. Am. Chem. Soc. 104 2797-2803, 1982. 

The fluoride ion is the least polarizable anion. It is small, having a diameter of 0.136 nm, 0.045 nm smaller than the chloride ion. The isoelectronic E and ions are the only anions of comparable size to many cations. These anions are about the same size as K" and Ba " and smaller than Rb" and Cs". The small size of E allows for high coordination numbers and leads to different crystal forms and solubiUties, and higher bond energies than are evidenced by the other haUdes. Bonds between fluorine and other elements are strong whereas the fluorine—fluorine bond is much weaker, 158.8 kj/mol (37.95 kcal/mol), than the chlorine—chlorine bond which is 242.58 kJ/mol (57.98 kcal/mol). This bond weakness relative to the second-row elements is also seen ia 0-0 and N—N single bonds and results from electronic repulsion. 

Molybdenum because of its unique chemical versatility and unusually high bio-availability has been incorporated widely into biological systems. It is the only second-row transition metal that is essential for most of living organisms and belongs to elements (along with Cu, Cd, Hg, Pb and Cr) potentially hazardous to humans. 

Figure 1.35 shows the second-row elements Li through F in their compounds with hydrogen. Note the transformation of hydrogen from hydride in LiH to a much smaller, protonlike entity in HF as the electronegativity of the heavier atom increases. 

Whereas the barrier for pyramidal inversion is low for second-row elements, the heavier elements have much higher barriers to inversion. The preferred bonding angle at trivalent phosphorus and sulfur is about 100°, and thus a greater distortion is required to reach a planar transition state. Typical barriers for trisubstituted phosphines are BOSS kcal/mol, whereas for sulfoxides the barriers are about 35-45 kcal/mol. Many phosphines and sulfoxides have been isolated in enantiomerically enriched form, and they undergo racemization by pyramidal inversion only at high temperature. ... 

Second-row elements, particularly phosphorus and sulfur, stabilize adjacent carba-nions. The pATs of some pertinent compounds are given in Table 7.10. 

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