Elements, 2, 5-7 actinide series transition

Ernest O. Lawrence, inventor of the cyclotron) This member of the 5f transition elements (actinide series) was discovered in March 1961 by A. Ghiorso, T. Sikkeland, A.E. Larsh, and R.M. Latimer. A 3-Mg californium target, consisting of a mixture of isotopes of mass number 249, 250, 251, and 252, was bombarded with either lOB or IIB. The electrically charged transmutation nuclei recoiled with an atmosphere of helium and were collected on a thin copper conveyor tape which was then moved to place collected atoms in front of a series of solid-state detectors. The isotope of element 103 produced in this way decayed by emitting an 8.6 MeV alpha particle with a half-life of 8 s. 

As regards the transition elements, the first row in particular show some common characteristics which define a substantial part of their chemistry the elements of the lanthanide and actinide series show an even closer resemblance to each other. 

The first (inconclusive) work bearing on the synthesis of element 104 was published by the Dubna group in 1964. However, the crucial Dubna evidence (1969-70) for the production of element 104 by bombardment of 94PU with loNe came after the development of a sophisticated method for rapid in situ chlorination of the product atoms followed by their gas-chromatographic separation on an atom-by-atom basis. This was a heroic enterprise which combined cyclotron nuclear physics and chemical separations. As we have seen, the actinide series of elements ends with 103 Lr. The next element should be in Group 4 of the transition elements, i.e. a heavier congenor of Ti, Zr and Hf. As such it would be expected to have a chloride... 

Krebs, Robert E. The history and use of our earth s chemical elements a reference guide. Westport (CT) Greenwood P, 1998. ix, 346p. ISBN 0-313-30123-9 A short history of chemistry — Atomic structure The periodic table of the chemical elements — Alkali metals and alkali earth metals - Transition elements metals to nonmetals — Metallics and metalloids - Metalloids and nonmetals — Halogens and noble gases - Lanthanide series (rare-earth elements) — Actinide, transuranic, and transactinide series... 

Transition metah—found in the groups located in the center of the periodic table, plus the lanthanide and actinide series. They are all solids, except mercury, and are the only elements whose shells other than their outer shells give up or share electrons in chemical reactions. Transition metals include the 38 elements from groups 3 through 12. They exhibit several oxidation states (oxidation numbers) and various levels of electronegativity, depending on their size and valence. 

Symbol Am Atomic Number 95 Atomic Weight 243.0614 an inner-transition, actinide series, radioactive man-made element electron configuration ... 

Symbol Lr atomic number 103 atomic weight 262 a transuranium inner-transition actinide series element a synthetic radioactive element electron configuration [RnjTs b/ Sdi valence +3 six isotopes of masses 255 to 260 have been synthesized longest-lived known isotope Lr-260 has half-life of 3 minutes. 

Symbol Md atomic number 101 atomic weight (most stable isotope) 257 a man-made radioactive transuranium element an inner-transition element of actinide series electron configuration [Rn]5/i37s2 valence +2, -i-3. Isotopes, half-lives and their decay modes are ... 

Symbol Pa atomic number 91 atomic weight 231.04 an actinide series radioactive element an inner-transition metal electron configuration [Rn]5/26di7s2 valence states +4 and +5 atomic radius 1.63A (for coordination number 12) twenty-two isotopes are known in the mass range 215-218,... 

Most transition metals of the three d-series in all their valency states exhibit ionic radii within the limits of 0.55 and 0.86 A, favourable to octahedral coordination. In fact higher coordination numbers are observed only in fluorides of the largest transition ions, above all in compounds of the lanthanide and actinide series. Therefore fluorides of those elements, though sometimes isostructural with compounds of the d-series, will not be discussed here. For information the books and reviews written by Spedding and Daane (291), Katz and Seaborg (181) and Kaiz and Sheft (182) may be consulted. 

Trifluorides are known from several elements of all transition series. In most of the compounds there are again linked MeFe-octahedra. A C.N. of 8 + 1 is found in the orthorhombic YFs-type [347) that is adapted by some of the rare earths trifluorides and will not be discussed here. The LaFa-type which represents the structure of trifluorides of the actinides, does not display a C.N. of 5, as was reported earlier A reinvestigation 220, 348) proved also 9-coordination for the larger Me-ions. 

The remaining exceptions concern the lanthanide series, where samarium at room temperature has a particular hexagonal structure and especially the lower actinides uranium, neptunium, and plutonium. Here the departure from simple symmetry is particularly pronounced. Comparing these three elements with other metals having partly filled inner shells (transition elements and lanthanides), U, Pu, Np have the lowest symmetry at room temperature, normal pressure. This particular crystallographic character is the reason why Pearson did not succeed to fit the alpha forms of U, Pu, and Np, as well as gamma-Pu into his comprehensive classification of metallic structures and treated them as idiosyncratic structures . Recent theoretical considerations reveal that the appearance of low symmetries in the actinide series is intimately linked to the behaviour of the 5f electrons. 

Recently a so called Nearly Periodic Table of Transition Elements has been published that depicts the intermediate position of the actinides between the 4f and 3d series. Figure 1 shows this table. The crossover between electron bonding behaviour of the unsaturated shells and the formation of magnetic moments in atoms occurs at the beginning and near the end of the 4f and 3 d series, respectively. In actinides, it occurs near the half filling of the actinide 5f shell, i.e. at the middle of the actinide series. 

The exact position of the crossover depends however, on the type of compound that is formed from an actinide. It appears usually for the radioactive heavier actinides. In order to study it, therefore, compoimds of Pu, Am, Cm have to be investigated. Compounds of elements preceding them in the actinide series present properties due to the itineracy of the 5 f electrons, which are somewhat similar to the d-transition elements compounds (especially 3d). Heavier actinides are lanthanide-like although their properties may depart from a true lanthanide behaviour unfortunately, their rarity and the difficulty of their handling is such that very few photoemission results are available for them. 

Within the actinide series Pu is the most intriguing element. On the basis of the Hubbard model, and taking into account an unhybridized bandwidth Wf (due only to f-f overlapping), the Un/Wf ratio is 0.7 for U and 3 for Pu in fact, one would have expected already for Pu a 5 f electron localization, since Uh > Wf. However, a hybridization of 5 f with (6d7s) states broadens the 5f bandwidth and delays the Mott-like transition (see Chap. A) from Pu to Am This influences many properties of Pu metaf . ... 

The main group metals are the most important, given the role of Na+, K+, and Ca + in bioelectrical excitability. The transition metals also have biological relevance. A formal definition of transition metals is that they have partially filled d ox f orbitals in either their free (uncombined) atoms or one or more of their ions. Transition metals may be divided into t/-block and /-block elements the /-block is further divided into the lanthanide and actinide series. Since/-block metals are not of great significance to medicinal... 

U is a member of the actinide series of elements which, together with the rare earths and the transition elements, possess a high heat of oxidation, a low oxide density compared with that of the metal, and the presence of an unfilled d shell in its electronic structure. While the reasons for the high pyrophoric potential of U are not clearly understood, they are thought to be related to these aforementioned properties (see under Pyrotechnics in Vol 8, P511 and Pyrophoric Incendiary Agents , P503-L)... 

Tucked into the periodic table between lanthanum (atomic number 57) and hafnium (atomic number 72) are the lanthanides. In this series of 14 metallic elements, the seven 4/orbitals are progressively filled, as shown in Figure 5.17 (page 185). Following actinium (atomic number 89) is a second series of 14 elements, the actinides, in which the 5f subshell is progressively filled. The lanthanides and actinides together comprise thef-block elements, or inner transition elements. 

Elements with atomic numbers ranging from 90 to 103, the actinides, are members of a transition series in which the first member is actinum (atomic number 89). They are analogues to lanthanides and occupy the same part of the Periodic Table at the next period. Only four of them have been found in nature the others are manmade elements produced by neutron irradiation or heavy-ion bombardment. All of them are radioactive [282]. Technetium (element 43), although not part of the actinides series, possesses two radioactive isotopes with long half-lives "Tc (2.12 x 105s, has the practical use) and 98Tc (1.5 x 106 years, a rhenium analogue) [283],... 

The rules above gave maximum and minimum oxidation numbers, but those might not be the only oxidation numbers or even the most important oxidation numbers for an element. Elements of the last six groups of the periodic table, for example, may have several oxidation numbers in their compounds, most of which vary from one another in steps of 2. For example, the major oxidation states of chlorine in its compounds are -1, +1, +3, +5, and +7. The transition metals have oxidation numbers that may vary from one another in steps of 1. The inner transition elements mostly form oxidation states of +3, but the first part of the actinide series acts more as transition elements and the elements have maximum oxidation numbers that increase from +4 for Th to +6 for U. These generalizations are not absolute rules, but allow students to make educated guesses about possible compound formation without exhaustive memorization. These possibilities are illustrated in Fig. 14-1. 

Valence electrons in atoms and molecules have a finite (albeit small) probability of being close to the nuclei and they can as a consequence acquire high instantaneous velocities.In fact,the velocities for the valence electrons can approach that of light as they pass in close proximity to heavier nuclei with Z >72.It is for this reason not too surprising that relativistic effects become of importance for the chemical properties of compounds containing 5d-block elements in the third transition series or 5f-block elements in the actinide series. 

The f-block elements comprise two series of inner transition elements which appear, firstly after lanthanum and secondly after actinium, in the Periodic Table. The elements from cerium to lutetium are known as the lanthanides and, because of its chemical similarity to these elements, lanthanum is usually included with them. Scandium and yttrium also show strong chemical similarities to the lanthanides, so that the chemistry of these elements is also often considered in conjunction with that of the lanthanide series. The second series of f-block elements, from thorium to lawrencium, is known as the actinide series and again it is usual to consider actinium together with this series. 

---