that both types use the same kind of outer shell orbitals, principally s and p, in bonding. Consequently both tend to form the same kinds of compounds and comparison can usefully be made. Table 1.
Comoarison of Calcium and Zinc
Electronic structure Radius (A): metallic poy~lent lonlc Ionization energy (kcal/mole): first average of two Eleetranegativity Melting point ( T ) Heat of fusion (keel/male) Boiling point ( T ) Heat of vaporization (keal/mole) Heat of atomization (kcal/rnole) Electrical conductance
Ca
Zn
2-8-8-2
2-8-18-2
Transition Groups
Group number. The group number represents the number of electrons in each atom that are beyond the nearest lower rare gas structure. Since this is the number that determines the bonding characteristics of the element, its use is closely analogous to that of the major groups in which t,he number is not only the number of outermost shell electrons, but a t the same time also the number of electrons beyond the nearest lower rare-gas structure or rlosed 1Bshell structure. The T designates the transition series. The iron, cobalt, and nickel groups are appropriately numbered T8, TQ, and T10. The copper, silver, and gold group now becomes Group T11, since no matter how the electrons are distributed in the atoms, copper and silver atoms each have eleven electrons beyond the nearest lower rare gas structure. (Gold has 25 because of the interposition of the lanthanides.) There are no transition groups numbered T1 or T 2 because no transition elements have only one or two electrons beyond the nearest lower rare gas shell structure. Inner transition elements. The lanthanide and actinide elements, which present a transition within a transition, electronically, by the interruption of the d orbital filling in order to fill the underlying f orbitals, are most conveniently studied as two separate series and are therefore so segregated in the periodic chart. This vertical separation is in recognition of the well known chemical differences between the lanthanides and actinides as shown by the prevalence of higher oxidation states in compounds of the latter, especially of the first few members. Such differences are not necessarily a reflection of differences in ground state electronic configurations of the two series but probably result from the small energy differences between 5f and 6d orbitals in contrast to the larger energies required to shift electrons from 4f to 5d orbitals. It seems reasonable to define both series of these elements as those containing incompletely fiUed f orbitals in the shell underlying the penultimate. By this definition there are only 13 instead of 14, the 14th, lutetium, having filled f orbitals.
Lutetium and lawrencium. Although the change from inner transition element to transition element, defining the former as above, is not nearly as great as that from transition to 18-shell element, the situation here electronically is somewhat similar. Lutetium has no more reawn to be classed with the rare earth elements electronically than does lanthanum. For this reason, lutetium is placed in the same transition Group T3 with lanthanum rather than with the inner transition elements below. Presumably lawrencium can be treated in the same manner. Table 2 lists some properties of lanthanum and lutetium and their compounds that suggest the nature of the changes brought about by increasing the nuclear charge while filling in the f orbitals. It can be seen that lutetium, although higher melting than lanthanum, is somewhat more volatile and forms weaker bonds to chlorine and iodine. Table 2.
Comparison of Lanthanum and Lutetium
Electronic structure Radius (A): rnetaliii ionic (Ma+) Density (g/ml) Melting paint ('C) Boiling point (OC) TrieNoride, heat of formation (kcd/l/equiv) triiodide, heat of formation (keal/equiv)
La.
Lu
2-&18-18-9-2
2-8-18-32-9-2
The Complete Long Form
At the bottom of the figure is placed an assembled periodic table, the complete "long form" in its true meaning. Students can learn from this the order of filling in of electrons as the elements are built up in order of increasing atomic number. They can also see how some of the awkwardness of the conventional long form of the periodic table has been eliminated by separating the chemical elements into three blocks.
Prof. A. W. Cordes sends this photograph of an illuminated wall chart in use at the University of Arkansas, Fayetteville. Cost is moderate, ($270.00 excluding labor); materials used are readily available. Symbols can be lit individually. Construction dehils are available for interested readers who request them from Prof. Cordes.
Volume 4 1, Number 4, April
1964
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189
