Noble-gas structure

For the transition metals it is often impossible to reach a noble gas structure except in covalent compounds (see effective atomic number rule) and it is found that relative stability is given by having the sub-shells (d or f) filled, half-filled or empty. 

The elements (X) in this group are two electrons short of a noble gas structure which they can achieve either by gaining or sharing electrons. The formation of the ion may require considerable amounts of energy thus for oxygen 650 kJ must be supplied for the reaction... 

Chromium forms a white solid, hexacarhonyl, Cr(CO)j, with the chromium in formal oxidation state 0 the structure is octahedral, and if each CO molecule donates two electrons, the chromium attains the noble gas structure. Many complexes are known where one or more of the carbon monoxide ligands are replaced by other groups of ions, for example [CrfCOlsI] . 

Two other ions that have noble-gas structures are... 

Several metals that are farther removed from the noble gases in the periodic table form positive ions. These include the transition metals in Groups 3 to 12 and the post-transition metals in Groups 13 to 15. The cations formed by these metals typically have charges of +1, +2, or +3 and ordinarily do not have noble-gas structures. We will postpone to Chapter 4 a general discussion of the specific charges of cations formed by these metals. 

Noble-gas structures are stable in molecules, as they are in atoms and ions. 

These examples illustrate the principle that atoms in covalently bonded species tend to have noble-gas electronic structures. This generalization is often referred to as the octet rule. Nonmetals, except for hydrogen, achieve a noble-gas structure by sharing in an octet of electrons (eight). Hydrogen atoms, in molecules or polyatomic ions, are surrounded by a duet of electrons (two). 

For example, whichever form of the table is used, an interesting feature emerges The sequence 2,10,18, 36,54, 86 of atomic numbers, in which each period is dosed in the sense of reaching a noble-gas structure, does not appear to have a strictly quantum-mechanical explanation. Although Pauli s brilliant discovery... 

L. Pauling, Proc. Roy. Soc., A114, 181 (1927). In this paper it was stated that the consideration of polarization of the anion by the cation would be expected to reduce the calculated values a number of facts, however, indicate that with ions with the noble gas structure the effect of polarization on the equilibrium distance is small. 

Manganese forms a decacarbonyl Mn2(CO)10 in which each manganese has the required share in 18 electrons to achieve the noble gas configuration. Reduction of this covalent compound with sodium amalgam gives the salt Na[Mn(CO)5], sodium pentacarbonyl-manganate ( - 1) in the ion Mn(CO)5 the noble gas structure is again attained. 

The electrostatic contribution is highly influenced by the dielectric constant of the medium since no relationship is found between the dielectric constant and the half-wave potential (not even for ions with noble gas structure, such as K ), it may again be concluded that covalent contributions should not be neglected in any solvation phenomena. 

A structure in which all the atoms have the noble gas structure is particularly stable. 

The increased stability of 4n + 2 cyclic planar polyenes, relative to their imaginary classical counterparts, comes about because all the bonding energy levels within the ji-system are completely filled. For benzene and pyridine there are three such levels, each containing two spin-paired electrons. There is then an analogy between the electronic constitutions of these molecules and atoms that achieve noble gas structure. 

Without further data, it should thus be possible to make some predictions regarding sizes of ions, and for this purpose we will confine our attention to ions with noble-gas structures. In any period the radii of the ions must decrease with increasing positive charge, as in the following series ... 

The heat of formation per equivalent increases when a positive ion, with a noble-gas structure, is replaced in a compound by another ion of equal charge, but greater radius. 

The previous discussion has been limited to ions with noble-gas structure the same phenomena occur in the highly-charged elements of the subgroups, as can be seen from the values of the heats of formation of some of the oxides, Table XIX> where again the... 

This ionic reaction is an abnormal one because the Cu2+ ion does not possess a noble-gas structure. 

The differing stability of various complex ions has just been discussed. As regards the dependence of stability on the central ion, the rules can be formulated very easily. When the central ion has a high charge, a small radius and not a noble-gas structure, the stability will be high because the attraction between the central and surrounding ions is then as large as it can be. 

The enthalpy of atomization of copper does not differ at all for the two compounds, and the atomization of chlorine adds only a small difference for the second mole of chlorine. The major energy cost for CuCl2 is the second ionization energy of copper which is compensated by the electron affinity to form the second chloride ion and especially the lattice energy. Since the electron ionized to form Cu2 is a d electron and does not break a noble gas structure, IE2 is not excessive, and both CuCl and CuCl2 are stable compounds. 

The general chemistry of Ac3 in both solid compounds and solution, where known, is very similar to that of lanthanum, as would be expected from the similarity in position in the Periodic Table and in radii (Ac3, 1.10 La3, 1.06 A) together with the noble gas structure of the ion. Thus actinium is a true member of Group 3, the only difference from lanthanum being in the expected increased basicity. The increased basic character is shown by the stronger absorption of the hydrated ion on cation-exchange resins, the poorer extraction of the ion from concentrated nitric acid solutions by tributyl phosphate, and the hydrolysis of the trihalides with water vapor at 1000°C to the oxohalides AcOX the lanthanum halides are hydrolyzed to oxide by water vapor at 1000°C. 

Similarly the atoms of group II of the periodic table can, by losing two electrons, produce ions with the noble-gas structures these ions are Be+, Mg++, Ca++, Sr -and Ba++. The alkaline-earth elements are hence bipositive in valence. The elements of group III are terpositive, those of group VI are binegative, etc. 

By reference to the periodic table find which ions in these compounds do not have a noble-gas structure, and underline them in the formulas. 

---