Fermium

SYMBOL Fm PERIOD 7 SERIES NAME Actinides ATOMIC NO 100 

ATOMIC MASS 257 amu VALENCE 3 OXIDATION STATE -t-3 NATURAL STATE Solid 

ORIGIN OF NAME Named after and to honor the scientist Enrico Fermi. 

ISOTOPES There are a total of 21 isotopes of fermium. Their half-lives range from fer-mium-258 s 370 microseconds to fermium-257 s 100.5 days, which is the longest of all its isotopes. None of fermium s isotopes exist in nature. All are artificially produced and are radioactive. 

Energy Levels/Shells/Electrons Orbitals/Electrons 

---

The use of larger particles in the cyclotron, for example carbon, nitrogen or oxygen ions, enabled elements of several units of atomic number beyond uranium to be synthesised. Einsteinium and fermium were obtained by this method and separated by ion-exchange. and indeed first identified by the appearance of their concentration peaks on the elution graph at the places expected for atomic numbers 99 and 100. The concentrations available when this was done were measured not in gcm but in atoms cm. The same elements became available in greater quantity when the first hydrogen bomb was exploded, when they were found in the fission products. Element 101, mendelevium, was made by a-particle bombardment of einsteinium, and nobelium (102) by fusion of curium and the carbon-13 isotope. 

The chemical properties of fermium have been studied solely with tracer amounts. In normal aqueous media, only the (111) oxidation state appears to exist. 

Sixteen isotopes of fermium are known to exist. 257Fm, with a half-life of about 100.5 days, is the longest lived. 250Fm, with a half-life of 30 minutes, has been shown to be a decay product of element 254-102. Chemical identification of 250Fm confirmed the production of element 102 (nobelium). 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

It is possible to prepare very heavy elements in thermonuclear explosions, owing to the very intense, although brief (order of a microsecond), neutron flux furnished by the explosion (3,13). Einsteinium and fermium were first produced in this way they were discovered in the fallout materials from the first thermonuclear explosion (the "Mike" shot) staged in the Pacific in November 1952. It is possible that elements having atomic numbers greater than 100 would have been found had the debris been examined very soon after the explosion. The preparative process involved is multiple neutron capture in the uranium in the device, which is followed by a sequence of beta decays. Eor example, the synthesis of EM in the Mike explosion was via the production of from followed by a long chain of short-Hved beta decays,... 

Isotopes sufficiently long-Hved for work in weighable amounts are obtainable, at least in principle, for all of the actinide elements through fermium (100) these isotopes with their half-Hves are Hsted in Table 2 (4). Not all of these are available as individual isotopes. It appears that it will always be necessary to study the elements above fermium by means of the tracer technique (except for some very special experiments) because only isotopes with short half-Hves are known. 

The effects of a rather distinct deformed shell at = 152 were clearly seen as early as 1954 in the alpha-decay energies of isotopes of californium, einsteinium, and fermium. In fact, a number of authors have suggested that the entire transuranium region is stabilized by shell effects with an influence that increases markedly with atomic number. Thus the effects of shell substmcture lead to an increase in spontaneous fission half-Hves of up to about 15 orders of magnitude for the heavy transuranium elements, the heaviest of which would otherwise have half-Hves of the order of those for a compound nucleus (lO " s or less) and not of milliseconds or longer, as found experimentally. This gives hope for the synthesis and identification of several elements beyond the present heaviest (element 109) and suggest that the peninsula of nuclei with measurable half-Hves may extend up to the island of stabiHty at Z = 114 andA = 184. 

Fermium, Fm Workers at Berkeley, 1952 Found in debris of first Enrico Fermi... 

Fermi lived only a little more than a decade after his hour of triumph. He spent most of this time at the University of Chicago, where, as in Rome, he surrounded himself with a group of outstanding gi aduate students, many of whom also later received Nobel Prizes. Fermi died of stomach cancer in 1954, but his name remains attached to many of the important contributions he made to physics. For example, element 100 is now called Fermium. 

Half-lives can be interpreted in terms of the level of radiation of the corresponding isotopes. Uranium has a very long half-life (4.5 X 109 yr), so it gives off radiation very slowly. At the opposite extreme is fermium-258, which decays with a half-life of 3.8 X 10-4 s. You would expect the rate of decay to be quite high. Within a second virtually all the radiation from fermium-258 is gone. Species such as this produce very high radiation during their brief existence. 

Faraday, Michael, 66,501,588 Fats, 602 Fatty acids, 604 Fermentation, 14 Fermium-258,295 Fertilizer, 62-63,573... 

For example, the most stable isotope of element 100, fermium, has a half-life of only 4.5 days ... 

Fable, Lost Child in Woods, 3 Faraday, Michael, 237 Fats, 425 Fermentation, 426 Fermium, oxidation number, 414 Ferromanganese, 403 Fission, nuclear, 120, 419 Flea, 88... 

Self-Test 17.5A The decay constant for fermium-254 is 210 s. What mass of the isotope will be present if a sample of mass 1.00 xg is kept for 10. ms ... 

---