Mendelevium
Encyclopedia
Mendelevium is a synthetic element
with the symbol Md (formerly Mv) and the atomic number
101. A metallic radioactive transuranic element in the actinide
series, mendelevium is usually synthesized by bombarding einsteinium
with alpha particle
s. It was named after Dmitri Ivanovich Mendeleev, who created the Periodic Table
. Mendeleev's periodic system is the fundamental way to classify all the chemical elements. The name "mendelevium" was accepted by the International Union of Pure and Applied Chemistry
(IUPAC). On the other hand, the proposed symbol "Mv" submitted by the discoverers was not accepted, and the IUPAC selected "Md" instead.
in addition to the more characteristic (for actinide elements) tripositive (III) oxidation state, the latter being the more dominantly exhibited state in an aqueous solution (chromatography being the process used). Sometimes, mendelevium can even be shown to exhibit a monopositive (I) state. 256Md has been used to find out some of the chemical properties of this element while in an aqueous solution. There are no other known uses of mendelevium and only trace amounts of the element have ever been produced. Other isotope
s of mendelevium, all radioactive, have been discovered, with 258Md being the most stable with a two-month half-life (about 55 days). Other isotopes range from 248 to 258 mass numbers and half-lives from a few seconds to about 51 days. The original 256Md had a half-life of 87 minutes.
-active with a t½ of 5 minutes and the corresponding α-energy. No such α-activity was observed, but the 101 fraction showed spontaneous fission representing a t½ less than 3 hours. Because spontaneous fission was also observed in the fraction containing fermium, the α-bombardment of 253Einsteinium
(Es) produced 256Md. The latter underwent electron capture to become 256Fermium
(Fm), which then decayed by spontaneous fission. So 256Fm was produced by the decays of cyclotron-synthesized mendelevium.
metallic state, similar to europium
(Eu) and ytterbium
(Yb), rather than a trivalent one. Thermochromatographic studies conducted with trace amounts of Md concluded that Md forms a divalent metal. With the aid of empirical correlation method, a divalent metallic radius of (0.194 ± 0.010) nm has been estimated. The estimated enthalpy
of sublimation is in the range of 134-142 kJ/mol.
s was expected. The elution of Md just before Fm in the elution
sequence of the trivalent actinides from the cation-exchange resin column, confirmed this prediction. Afterwards, Md in the form of insoluble hydroxide
s and fluorides that are quantitatively coprecipitated with trivalent lanthanides was found. The cation-exchange resin column as well as the HDEHP solvent extraction column elution date is consistent with a trivalent state for Md and an ionic radius smaller than Fm. An ionic radius of 0.0192 nm and a coordination number of 6 for Md3+ was predicted using empirical correlations. Using the known ionic radii for the trivalent rare earths and the linear correlation of log distribution coefficient with ionic radius, an average ionic radius of 0.089 nm was estimated for Md3+ and a heat of hydration of – (3654 ± 12) kJ/mol calculated using empirical models and the Born-Haber cycle.
In reducing conditions, an anomalous chemical behavior of Md was found. Coprecipitation with BaSO4 and solvent extraction chromatography experiments using HDEHP were carried out in different reducing agents. These showed that Md3+ could easily be reduced to a stable Md2+ in aqueous solution. Mendelevium can also be reduced to the monovalent state in water-ethanol solutions. The cocrystallization of Md+ with salts of divalent ions is due to the formation of mixed crystals. For Md+, an ionic radius of 0.117 nm was found. The oxidation of Md3+ to Md4+ was rather unsuccessful.
, Glenn T. Seaborg
, Gregory R. Choppin, Bernard G. Harvey, and Stanley G. Thompson (team leader) in early 1955 at the University of California, Berkeley. The team produced 256Md (half-life
of 87 minutes) when they bombarded an 253Es
target with alpha particle
s (helium
nuclei) in the Berkeley Radiation Laboratory's 60-inch cyclotron
(256Md was the first isotope of any element to be synthesized one atom at a time). Element 101 was the ninth transuranic element synthesized. The first 17 atom
s of this element were created and analyzed using the ion-exchange adsorption-elution method. During the process, mendelevium behaved very much like thulium
, its naturally occurring homologue
.
The recoil target consisted of 10−9 of 253Es which were deposited electrolytically on a thin gold foil (also Be, Al and Pt can be used). It was bombarded by 41 eV α-particles in the Berkeley cyclotron with a very high beam density of 6∙1013 particles per second over an area of 0.05 cm2. The target was cooled by water or liquid helium. The use of helium, in a gaseous atmosphere, slowed down the recoil atoms. This gas could be pumped out of the reaction chamber through a small orifice to form a ‘gas-jet’. Some fraction of the nonvolatile product atoms carried along with the gas, were deposited permanently on the foil surface. The foil could be removed periodically and a new foil could be installed. The next reaction was used for the mendelevium discovery experiment:
253Es + 4He → 256Md + 1n
as eluant. To separate Md rapidly from the catcher foil, an anion-exchange chromatography using 6M HCl as eluant can be used. The gold remained on the column while the Md and other actinides passed through. A final isolation of Md3+ from other trivalent actinides was also required. To separate fractions containing elements 99, 100 and 101, a cation-exchange resin column (Dowex-50 exchange column) treated with ammonium salts was used. A chemical identification was made on the basis of its elution position just before Fm. In series of repetitive experiments, they made use of the eluant: α-hydroxyisobutyrate solution (α-HIB). Using the ‘gas-jet’ method, the first two steps can be eliminated. There was shown that in this method it is possible to transport and collect individual product atoms in a fraction of a second some tens of meters away from the target area. Effective transport over long distances requires the presence of large clusters (KCl aerosols) in the ‘carrier’ gas. It is used frequently in the production and isolation of transeinsteinium elements.
Another possible way to separate the 3+ actinides can be achieved by solvent extraction chromatography using bis-(2-ethylhexyl) phosphoric acid (abbreviated as HDEHP) as the stationary organic phase and HNO3 as the mobile aqueous phase. The actinide elution sequence is reversed from that of the cation-exchange resin column. The Md separated by this method has the advantage to be free of organic complexing agent compared to the resin column. The disadvantage of this method is that Md elutes after Fm late in the sequence.
of 51.5 days, 260Md with a half-life of 31.8 days, and 257Md with a half-life of 5.52 hours. All of the remaining radioactive isotopes have half-lives that are less than 97 minutes, and the majority of these have half-lives that are less than 5 minutes. This element also has 5 meta states, with the longest-lived being 258mMd (t½ = 58 minutes). The isotopes of mendelevium range in atomic weight
from 245.091 u
(245Md) to 262.109 u (262Md).
Synthetic element
In chemistry, a synthetic element is a chemical element that is too unstable to occur naturally on Earth, and therefore has to be created artificially. So far 30 synthetic elements have been discovered—that is, synthesized...
with the symbol Md (formerly Mv) and the atomic number
Atomic number
In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
101. A metallic radioactive transuranic element in the actinide
Actinide
The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
series, mendelevium is usually synthesized by bombarding einsteinium
Einsteinium
Einsteinium is a synthetic element with the symbol Es and atomic number 99. It is the seventh transuranic element, and an actinide.Einsteinium was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Albert Einstein...
with alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
s. It was named after Dmitri Ivanovich Mendeleev, who created the Periodic Table
Periodic table
The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
. Mendeleev's periodic system is the fundamental way to classify all the chemical elements. The name "mendelevium" was accepted by the International Union of Pure and Applied Chemistry
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...
(IUPAC). On the other hand, the proposed symbol "Mv" submitted by the discoverers was not accepted, and the IUPAC selected "Md" instead.
Characteristics
Researchers have shown that mendelevium has a moderately stable dipositive (II) oxidation stateOxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
in addition to the more characteristic (for actinide elements) tripositive (III) oxidation state, the latter being the more dominantly exhibited state in an aqueous solution (chromatography being the process used). Sometimes, mendelevium can even be shown to exhibit a monopositive (I) state. 256Md has been used to find out some of the chemical properties of this element while in an aqueous solution. There are no other known uses of mendelevium and only trace amounts of the element have ever been produced. Other isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s of mendelevium, all radioactive, have been discovered, with 258Md being the most stable with a two-month half-life (about 55 days). Other isotopes range from 248 to 258 mass numbers and half-lives from a few seconds to about 51 days. The original 256Md had a half-life of 87 minutes.
Radioactivity
The radioactivity of the trivalent element, mendelevium, is definitely noteworthy. It was expected that the reaction would be253Es (α,n) 255Md, where 255Md was αAlpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
-active with a t½ of 5 minutes and the corresponding α-energy. No such α-activity was observed, but the 101 fraction showed spontaneous fission representing a t½ less than 3 hours. Because spontaneous fission was also observed in the fraction containing fermium, the α-bombardment of 253Einsteinium
Einsteinium
Einsteinium is a synthetic element with the symbol Es and atomic number 99. It is the seventh transuranic element, and an actinide.Einsteinium was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Albert Einstein...
(Es) produced 256Md. The latter underwent electron capture to become 256Fermium
Fermium
Fermium is a synthetic element with the symbol Fm. It is the 100th element in the periodic table and a member of the actinide series. It is the heaviest element that can be formed by neutron bombardment of lighter elements, and hence the last element that can be prepared in macroscopic quantities,...
(Fm), which then decayed by spontaneous fission. So 256Fm was produced by the decays of cyclotron-synthesized mendelevium.
The metallic state
Johansson and Rosengren predicted in 1975 that Md would prefer a divalentDivalent
In chemistry, a divalent ion or molecule has a valence of two and thus can form two bonds with other ions or molecules. An older term for divalent is bivalent....
metallic state, similar to europium
Europium
Europium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...
(Eu) and ytterbium
Ytterbium
Ytterbium is a chemical element with the symbol Yb and atomic number 70. A soft silvery metallic element, ytterbium is a rare earth element of the lanthanide series and is found in the minerals gadolinite, monazite, and xenotime. The element is sometimes associated with yttrium or other related...
(Yb), rather than a trivalent one. Thermochromatographic studies conducted with trace amounts of Md concluded that Md forms a divalent metal. With the aid of empirical correlation method, a divalent metallic radius of (0.194 ± 0.010) nm has been estimated. The estimated enthalpy
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
of sublimation is in the range of 134-142 kJ/mol.
Solution chemistry
Before the actual discovery of mendelevium, the trivalent state was the most stable one in aqueous solution. Accordingly, a similar chemical behavior to the other 3+ actinides and lanthanideLanthanide
The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
s was expected. The elution of Md just before Fm in the elution
Elution
Elution is a term used in analytical and organic chemistry to describe the process of extracting one material from another by washing with a solvent ....
sequence of the trivalent actinides from the cation-exchange resin column, confirmed this prediction. Afterwards, Md in the form of insoluble hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
s and fluorides that are quantitatively coprecipitated with trivalent lanthanides was found. The cation-exchange resin column as well as the HDEHP solvent extraction column elution date is consistent with a trivalent state for Md and an ionic radius smaller than Fm. An ionic radius of 0.0192 nm and a coordination number of 6 for Md3+ was predicted using empirical correlations. Using the known ionic radii for the trivalent rare earths and the linear correlation of log distribution coefficient with ionic radius, an average ionic radius of 0.089 nm was estimated for Md3+ and a heat of hydration of – (3654 ± 12) kJ/mol calculated using empirical models and the Born-Haber cycle.
In reducing conditions, an anomalous chemical behavior of Md was found. Coprecipitation with BaSO4 and solvent extraction chromatography experiments using HDEHP were carried out in different reducing agents. These showed that Md3+ could easily be reduced to a stable Md2+ in aqueous solution. Mendelevium can also be reduced to the monovalent state in water-ethanol solutions. The cocrystallization of Md+ with salts of divalent ions is due to the formation of mixed crystals. For Md+, an ionic radius of 0.117 nm was found. The oxidation of Md3+ to Md4+ was rather unsuccessful.
History
Mendelevium (for Dimitri Ivanovich Mendeleev, surname commonly transliterated into Latin script as Mendeleev, Mendeleyev, Mendeléef, or even Mendelejeff, and first name sometimes transliterated as Dmitry or Dmitriy) was first synthesized by Albert GhiorsoAlbert Ghiorso
Albert Ghiorso was an American nuclear scientist and co-discoverer of a record 12 chemical elements on the periodic table. His research career spanned five decades, from the early 1940s to the late 1990s.-Early life:...
, Glenn T. Seaborg
Glenn T. Seaborg
Glenn Theodore Seaborg was an American scientist who won the 1951 Nobel Prize in Chemistry for "discoveries in the chemistry of the transuranium elements", contributed to the discovery and isolation of ten elements, and developed the actinide concept, which led to the current arrangement of the...
, Gregory R. Choppin, Bernard G. Harvey, and Stanley G. Thompson (team leader) in early 1955 at the University of California, Berkeley. The team produced 256Md (half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 87 minutes) when they bombarded an 253Es
Einsteinium
Einsteinium is a synthetic element with the symbol Es and atomic number 99. It is the seventh transuranic element, and an actinide.Einsteinium was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Albert Einstein...
target with alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
s (helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
nuclei) in the Berkeley Radiation Laboratory's 60-inch cyclotron
Cyclotron
In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...
(256Md was the first isotope of any element to be synthesized one atom at a time). Element 101 was the ninth transuranic element synthesized. The first 17 atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s of this element were created and analyzed using the ion-exchange adsorption-elution method. During the process, mendelevium behaved very much like thulium
Thulium
Thulium is a chemical element that has the symbol Tm and atomic number 69. Thulium is the second least abundant of the lanthanides . It is an easily workable metal with a bright silvery-gray luster...
, its naturally occurring homologue
Homology (chemistry)
In chemistry, homology refers to the appearance of homologues. A homologue is a compound belonging to a series of compounds differing from each other by a repeating unit, such as a methylene group, a peptide residue, etcetera....
.
Discovery in detail
The discovery was based on a grand total of only 17 atoms. It is synthesized via the 253Es (α,n) 256101 reaction in the 60-Inch-Cyclotron (= 152 cm) at Berkeley (California). The target can be produced by irradiation of lighter isotopes as plutonium in the Materials Testing Reactor at the Arco Reactor Station in Idaho. Remarkable is that this target consisted of only 109 atoms of highly radioactive 253Es (with a half-life of 20.5 days). By elution through a calibrated cation exchange resin column, mendelevium was separated and chemically identified.Determining feasibility
To predict if this method would be possible, they made use of a rough calculation. The number of atoms that would be produced, would be approximately equal to the number of atoms of target material times its cross section times the ion beam intensity times the time of bombardment related to the half-life of the product when bombarding for a time of the order of its half-life). This gave 1 atom per experiment. Thus under optimum conditions, the preparation of only one atom of element 101 per experiment could be expected. This calculation demonstrated that it was feasible to go ahead with the experiment.Recoil technique
The actual synthesis was done by a recoil technique, introduced by Albert Ghiorso. In this technique, the target element was placed on the opposite side of the target from the beam and caught the recoiling atoms on a catcher foil. This recoil target was made by an electroplating technique, developed by Alfred Chetham-Strode. This technique gave a very high yield, which is absolutely necessary when working with such a rare product as the einsteinium target material.The recoil target consisted of 10−9 of 253Es which were deposited electrolytically on a thin gold foil (also Be, Al and Pt can be used). It was bombarded by 41 eV α-particles in the Berkeley cyclotron with a very high beam density of 6∙1013 particles per second over an area of 0.05 cm2. The target was cooled by water or liquid helium. The use of helium, in a gaseous atmosphere, slowed down the recoil atoms. This gas could be pumped out of the reaction chamber through a small orifice to form a ‘gas-jet’. Some fraction of the nonvolatile product atoms carried along with the gas, were deposited permanently on the foil surface. The foil could be removed periodically and a new foil could be installed. The next reaction was used for the mendelevium discovery experiment:
253Es + 4He → 256Md + 1n
Purification and isolation
The removal of the Md atoms from the collector foil was done by acid etching or total dissolution of the thin gold foil. They can be purified and isolated from other product activities by several techniques. Separation of trivalent actinides from lanthanide fission products and La carrier can be done by a cation-exchange resin column using a 90% water/10% ethanol solution saturated with HClHydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
as eluant. To separate Md rapidly from the catcher foil, an anion-exchange chromatography using 6M HCl as eluant can be used. The gold remained on the column while the Md and other actinides passed through. A final isolation of Md3+ from other trivalent actinides was also required. To separate fractions containing elements 99, 100 and 101, a cation-exchange resin column (Dowex-50 exchange column) treated with ammonium salts was used. A chemical identification was made on the basis of its elution position just before Fm. In series of repetitive experiments, they made use of the eluant: α-hydroxyisobutyrate solution (α-HIB). Using the ‘gas-jet’ method, the first two steps can be eliminated. There was shown that in this method it is possible to transport and collect individual product atoms in a fraction of a second some tens of meters away from the target area. Effective transport over long distances requires the presence of large clusters (KCl aerosols) in the ‘carrier’ gas. It is used frequently in the production and isolation of transeinsteinium elements.
Another possible way to separate the 3+ actinides can be achieved by solvent extraction chromatography using bis-(2-ethylhexyl) phosphoric acid (abbreviated as HDEHP) as the stationary organic phase and HNO3 as the mobile aqueous phase. The actinide elution sequence is reversed from that of the cation-exchange resin column. The Md separated by this method has the advantage to be free of organic complexing agent compared to the resin column. The disadvantage of this method is that Md elutes after Fm late in the sequence.
The first "Hooray!"
There was no direct detection, but by observation of spontaneous fission events arising from its electron-capture daughter 256Fm. These events were recorded during the night of February 19, 1955. The first one was identified with a "hooray" followed by a "double hooray" and a "triple hooray". The fourth one eventually officially proved the chemical identification of the 101st element, mendelevium. Additional analysis and further experimentation, showed the isotope to have mass 256 and to decay by electron capture with a half-life of 1.5 h.Isotopes
Sixteen isotopes of mendelevium from mass 245 to 260 have been characterized, with the most stable being 258Md with a half-lifeHalf-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 51.5 days, 260Md with a half-life of 31.8 days, and 257Md with a half-life of 5.52 hours. All of the remaining radioactive isotopes have half-lives that are less than 97 minutes, and the majority of these have half-lives that are less than 5 minutes. This element also has 5 meta states, with the longest-lived being 258mMd (t½ = 58 minutes). The isotopes of mendelevium range in atomic weight
Atomic weight
Atomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12...
from 245.091 u
Atomic mass unit
The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of...
(245Md) to 262.109 u (262Md).
Further reading
- Hoffman, D.C., Ghiorso, A., Seaborg, G. T. The transuranium people: the inside story, (2000), 201-229
- Morss, L. R., Edelstein, N. M., Fuger, J., The chemistry of the actinide and transactinide element, 3, (2006), 1630–1636
- Seaborg, G. T., Les elements tranuraniens artificiels, (1967), 39-45
- Gol’danskii, V. I., Polikanov, S. M., The transuranium elements, (1973), 101-103
- Seaborg, G.T., The transcalifornium elements. Journal of Chemical Education, (1959), 36, 38-44
- Guide to the Elements - Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1