Mendelevium

Mendelevium
Atomic number	101
Symbol	Md
Group	Actinides
Electronegativity	1.3 (Pauling)
Electron configuration	[Rn] 5f13 7s2
Discovery	1955; first atom-at-a-time ID via recoil (Ghiorso et al., UC Berkeley)
Period	7
Main isotopes	258Md, 257Md, 260Md
Phase STP	Solid
Block	f
Oxidation states	+3, +2
Wikidata	Q1898

Mendelevium – Element 101

Mendelevium (symbol Md, atomic number 101) is a synthetic, highly radioactive metal in the actinide series (Period 7, f-block). It is a transuranic element (meaning it follows uranium in atomic number) with no stable isotopes. All mendelevium in existence has been produced artificially in particle accelerators; it is not found naturally on Earth. The element is named after Dmitri Mendeleev, who devised the periodic table. Mendelevium’s chemistry is dominated by the +3 oxidation state (like other late actinides and lanthanides), with a notable +2 state accessible under reducing conditions. In everyday environments it would be a silvery-gray metallic solid (predicted by analogy with its neighbors), but only microscopic “one-atom-at-a-time” quantities have ever been made, making its physical appearance and bulk properties largely unmeasured. Mendelevium compounds are known only in tracer quantities in the laboratory. Its primary interest is scientific: studying the chemistry and nuclear physics of the heaviest elements.

Atomic Structure and Electron Configuration

Mendelevium atoms have 101 electrons. Their expected ground-state configuration is [Rn] 5f^13 7s^2, meaning all shells up to the radon core (atomic number 86) are filled, with 13 electrons in the 5f subshell and 2 in the 7s shell. Thus it has fifteen valence electrons outside the radon core, although only three of those (the 5f^13 7s^2 electrons) are easily removed in forming ions. The common +3 oxidation state corresponds to losing those three, leaving a [Rn] 5f^12 core (similar to other late actinides). The +2 state occurs when Md^3+ is partially reduced by one electron back to Md^2+, giving a [Rn] 5f^13 configuration. (Attempts to reach higher states like +4 have failed; strong oxidants do not convert Md^3+ beyond +3.)

Because it lies well down in the periodic table, mendelevium’s atomic radius is relatively small. The covalent radius has been estimated at about 173 picometers (1.73 Å). By comparison, lighter actinides in the late series (like americium, curium) have similar or slightly larger radii. Across the actinide series there is a gradual contraction in size, so Md(101) is smaller than its immediate predecessors. Its first ionization energy is relatively low – about 6.6 eV (about 635 kJ/mol) – reflecting the loosely held 7s electrons in this heavy atom. On the Pauling scale its electronegativity is around 1.3, similar to americium (1.3) and curium (1.28). These values place it as less electronegative than most transition metals but in line with other actinides/lanthanides. In summary, mendelevium’s outer electrons are strongly influenced by 5f–7s interactions: it behaves like a heavy metal with a moderately high polarizability and typical actinide electronic properties.

Periodic trends: As an actinide, Md follows the general trend of early f-block elements. Its 5f electrons are more localized than, say, the d electrons of transition metals, causing similar chemistry among the late actinides and lanthanides. Its atomic and ionic radii shrink slightly compared to lighter actinides (a phenomenon akin to the lanthanide contraction). Those actinide trends mean Md^3+ (ionic radius ~90 picometers for six coordinates) is comparable to Ho^3+ or Er^3+ in size. In periodic-group terms Mendelevium does not belong to a traditional “column” with stable elements; it is part of the series of actinides where +3 is dominant.

Isotopes and Nuclear Properties

All mendelevium isotopes are radioactive. No stable or long-lived primordial isotope exists. To date about sixteen isotopes have been identified, with mass numbers from A=245 up to A=260. The most long-lived is mendelevium-258 (¹²⁸Md) with a half-life of about 51.5 days. The next most persistent are ²⁶⁰Md (half-life ≈31.8 days) and ²⁵⁹Md (about 96 minutes). All others decay much faster: for example ²⁵⁷Md lives only about 5 minutes and ²⁵⁶Md about 1.5 hours. Thus effectively mendelevium vanishes within days of production.

The primary decay mode of odd-mass mendelevium isotopes (such as ²⁵⁹Md or ²⁵⁶Md) is electron capture (converting Md to fermium) or alpha emission. The even-mass isotopes (especially ²⁵⁸Md and ²⁶⁰Md) decay mostly by alpha emission; for example ²⁵⁸Md α-decays to ²⁵⁴Es (einsteinium) with a typical α-particle energy around 7.3 MeV. Spontaneous fission is a minor branch (especially for ²⁵⁸Md, with only trace probability). In each α-decay, a helium nucleus is emitted. The half-lives and decay energies have been measured by detecting these emissions. There are also a few nuclear isomers (metastable states) known, such as ²⁵⁸mMd, with different spin states that generally decay to the ground-state isotopes.

Because of their short half-lives and minute amounts, mendelevium isotopes have no practical use in radiometric dating or medical tracing. They do serve as valuable tracer isotopes in nuclear chemistry experiments. For example, tiny tracer amounts of ²⁵⁸Md can be used to follow the chemical behavior of element 101 under laboratory conditions. However, there is no industrial or environmental cycle for mendelevium: it is so rare and unstable that it never accumulates in nature.

Allotropes and Typical Compounds

No crystalline allotropes of metallic mendelevium have ever been produced in quantity, so its bulk solid structure is unknown. By analogy with other late actinides (like berkelium and californium, which are hexagonal close-packed at room temperature), Mendelevium metal is assumed to have an α-phase with hexagonal close packing under ambient conditions, possibly transforming to an fcc phase at high temperature. In any case, only single atoms or tiny clusters have been handled, so these expectations are entirely theoretical. If melt- or castable quantities could be made, Md would likely behave like a typical heavy actinide metal: silvery and metallic with good electrical and thermal conductivity.

Chemically, mendelevium falls squarely in line with the late-actinide (and lanthanide) pattern. The dominant oxidation state is +3, so the most common compounds are the mendelevium(III) species. For example, Md₂O₃ (mendelevium(III) oxide) would be the expected solid oxide (by analogy with europium(III) oxide or americium(III) oxide). Halides such as MdCl₃, MdBr₃ or MdI₃ are also expected, wherein Md^3+ is surrounded by six halide ions, forming colorless or pale compounds. In aqueous solution Md^3+ would exist as a complex ion like [Md(H₂O)₉]³⁺, similar to other trivalent actinides. Microtracer experiments have confirmed, for instance, that Mendelevium(III) oxalate and chloride can be precipitated under suitable conditions, just as for neighboring actinides.

The +2 oxidation state is unusual for heavy actinides but is accessible for mendelevium. Reduced Md(II) compounds (e.g. MdCl₂ or oxide MdO) have never been isolated in bulk, but in dilute aqueous solution Md^3+ can be chemically reduced to Md^2+ (for example with zinc metal or Eu^2+), a fact first demonstrated in specialized experiments. This parallels the situation in the lighter lanthanides (e.g. the stable +2 state of Eu, Yb) and confirms some similarities between actinide and lanthanide chemistry. There is no known stable +4 state for Md; strong oxidizers fail to pull off a fourth electron.

No organometallic complexes or extended molecules are known for Md (again due to the tiny amounts available). In general, bonding is expected to be largely ionic: Md^3+ acts like a hard Lewis acid. Like other actinides, the +3 ion will form coordination complexes with multidentate ligands (such as EDTA or diethylenetriaminepentaacetate) in analogy to lanthanide(III) chemistry. Studies have shown that careful ion-exchange or solvent extraction procedures (for example, using α-hydroxyisobutyric acid or ion-exchange resins) can separate Md^3+ from its neighboring actinides, a key technique used in its discovery. In summary, mendelevium forms typical late-actinide compounds (oxide, hydroxide, halides, salts) in the +3 state, and in very special cases forms divalent complexes when strongly reduced.

Physical Properties

Bulk physical data for mendelevium are largely theoretical. It is expected to be a metallic solid at standard conditions. Predictions based on periodic trends give a melting point around 1100 K (about 830 °C) and a boiling point on the order of magnitude of other actinides (likely well above 1700 K, though no measurement exists). Sources sometimes cite a melting point of 827 °C. The density is also estimated theoretically: one value given in element databases is about 10.3 g/cm³, but comparisons with neighboring actinides (e.g. berkelium at ~14 g/cm³) suggest it could be in the 13–15 g/cm³ range. In any case, it is a very dense material, as expected for a heavy element with high atomic mass 258] u).

Because of the inability to make bulk samples, key physical constants (heat capacity, thermal conductivity, etc.) are unknown. Mendelevium would conduct electricity and heat like a metal if it were in solid form, and like most actinides it would likely be paramagnetic (containing unpaired 5f electrons). Chemically, metal mendelevium would be highly reactive – it should rapidly oxidize in air and react with steam or acids. Its oxide and hydroxide are presumed to be refractory solids.

Spectroscopic data are extremely limited. No practical optical spectrum of Md atoms has been recorded, but atomic spectroscopy (suitable for identifying single atoms) would show lines similar to other f-block elements. Because it decays by α-emission, one could also think of its radiation giving characteristic X-rays as the atom decays, but again no detailed atomic spectra are used for identification in practice – detection is done via radiochemistry.

Chemical Reactivity and Trends

Mendelevium metal, were it observable, would be very reactive. It would ignite in air or oxygen to form the oxide (likely Md₂O₃ or MdO₂), and it would react with water or steam to give hydroxide (Md(OH)₃) and hydrogen gas, akin to the reactivity of reactive lanthanides and early actinides. In general, Md is lower on the reactivity series than alkali or alkaline earth metals, but among actinides it is quite active. A freshly prepared chunk of Md metal would tarnish quickly in air. No large-scale corrosion studies exist, but it would likely form a thin protective oxide layer on its surface (since many actinide metals do), slowing further corrosion under mild conditions.

In aqueous solution, mendelevium behaves like other trivalent actinides/lanthanides. Mendelevium(III) ions are strongly Lewis acidic: they form coordination complexes and precipitates. For example, adding a base to a solution of Md³⁺ will precipitate insoluble Md(OH)₃ (the trivalent oxide-hydroxide), which then dissolves in excess base as complex ions. Md³⁺ will form [Md(H₂O)_n]³⁺ aquo ions in acid, and will coordinate to ligands like nitrate, sulfate, or chloride much as Am³⁺ or Cm³⁺ do. There is no simple “acid” form of Md like an air-stable oxide; it primarily exists as Md³⁺ in solution at normal pH values.

The key redox chemistry is the presence of the +2 state. Mendelevium(III) can be reduced to Md²⁺ by strong reductants (such as zinc amalgam or divalent europium) in solution. The standard reduction potential for Md³⁺/Md²⁺ has been measured to be about +0.2 volts (vs. the standard hydrogen electrode), meaning Md³⁺ is easier to reduce than most lanthanides (with their Ln³⁺/Ln²⁺ around -0.2 to -0.4 V for Eu-Gd, for example) and is stable as Md²⁺. This divalent state in solution is one of Mendelevium’s unique chemical features – it was the first heavy actinide shown to form a stable M^(2+) ion in condensed matter. However, metallic Md²⁺ compounds (like a bulk MdCl₂ salt) have not been isolated. Oxidation beyond +3 is not observed: strong oxidizers (like sodium bismuthate) do not convert Md³⁺ to Md⁴⁺.

Overall, then, mendelevium’s chemistry is dominated by the +3 state (with characteristic trivalent salt and complex chemistry), with a notable but limited ability to attain +2 under reducing conditions. Its chemical behavior thus parallels the heavier lanthanides (where +3 is dominant, +2 appears for Eu, Yb) and the heavy actinides (Am, Cm, etc. all strongly favor +3).

Occurrence and Production

Mendelevium is essentially nonexistent in nature. It has never been found in the Earth’s crust, rocks, minerals, or ores. Tiny, undetectable amounts of Md might be created momentarily in the upper atmosphere by cosmic rays, but no natural reservoir or steady source exists. The element is exclusively man-made.

Production of mendelevium takes place only in nuclear research facilities. The classic and primary method is by particle bombardment in a cyclotron. For example, one well-established route is:

Einsteinium-253 (^253Es) + alpha particle (⁴He) → ^256Md + neutron.

In practice, a microgram or so of ^253Es is used as a target. A high-energy beam of helium nuclei (alpha particles) from a cyclotron is directed onto the einsteinium target. Each collision fuses 2 protons into the nucleus, producing mendelevium-256 (^256Md) and releasing a free neutron. Because only about one Md atom is created per hour under these conditions (even with a million Es atoms available), the yield is extremely low. Special “recoil catcher” techniques were developed: the einsteinium is deposited on a thin foil and when Md atoms are made, they recoil out of the target into a backing foil. Chemists then dissolve the foil and chemically separate the Md from other elements. (This recoil-catcher method was indeed how the element was first discovered.)

Modern technology allows similar production at very low rates: it is possible to make on the order of 10^6 mendelevium atoms per hour by bombarding about 1 microgram of Es-253 with 40 MeV alpha particles. However, even that high-tech rate yields only tiny “pico-gram” quantities. No industrial-scale production exists; the element is made only in mendelevium laboratories on demand for experiments. Examples of facilities that have produced Md include Lawrence Berkeley National Laboratory (California, USA, site of its original discovery), Oak Ridge National Laboratory (USA), the Joint Institute for Nuclear Research (Dubna, Russia), and other major nuclear centers with heavy-ion accelerators.

Other particle reactions can also yield Md isotopes. For instance, high-flux reactors can make einsteinium and then decay chains may produce Md. But direct neutron-capture routes fail beyond fermium (the so-called “fermium gap”), so a cyclotron is the practical route. All production involves sophisticated cyclotron or heavy-ion accelerators; there are no “ores” or mining methods for mendelevium.

Applications and Technology

Because of its extreme rarity and radioactivity, mendelevium has no practical applications in industry or medicine. No uses in energy, electronics, or materials are known. It is used only in basic scientific research.

The value of mendelevium lies in fundamental studies. Researchers use it as a testbed in nuclear physics and chemistry: for example, to examine how very heavy atoms behave, to calibrate detection equipment (such as alpha or neutron detectors), or to investigate actinide separation chemistry. The discovery of the +2 oxidation state in Md spurred theoretical work on electron shell models for actinides, and continues to inform our understanding of 5f-electron chemistry. Occasionally, experiments involving Md also touch on superheavy element research (for instance by using Md isotopes as starting material or calibration, or by studying its decay to neighboring elements).

In short, mendelevium’s only role is as a pure research novelty. It is briefly mentioned in textbooks and the periodic table as a curiosity of the transuranium series. Any “technology” involving Md is purely laboratory equipment (glove boxes, spectrometers, separators) used to detect and manipulate a handful of atoms. There are no catalysts, batteries, or alloys involving mendelevium.

Biology, Environment, and Safety

Mendelevium has no known biological role – no creatures, plants, or microbes use it. If it were somehow introduced into a living organism, its chemistry would resemble that of other heavy actinides or lanthanides: it would likely bind to proteins or bone (much like plutonium or americium do), but its intense radioactivity would quickly cause severe harm. In realistic terms, no biochemistry is relevant because no one can ingest or handle enough Md to interact biologically.

From an environmental standpoint, mendelevium does not cycle through nature. If an atom is released into the environment (as waste from a lab, for example), it will simply decay mostly by alpha emission and turn into other heavy elements (e.g. fermium, einsteinium) long before it spreads far. Acutely, as with any actinide, its chemical behavior would be to adsorb onto soils or sediments and accumulate in organisms that concentrate heavy elements, but again the quantities are so minute that there is effectively no environmental presence.

Safety is governed entirely by radioactivity. Mendelevium isotopes are powerful α- and β-emitters (and some spontaneous fission), so even a tiny amount carries a huge radiation dose. Accordingly, regulatory bodies have set extremely low exposure limits. For example, the International Commission on Radiological Protection (ICRP) estimated that the maximum annual ingestion of the longest-lived isotope ^258Md should be on the order of 10^6 becquerels, which corresponds to only a few nanograms (2.5×10^–9 g) of substance. Inhalation limits are even lower (on the order of 10^3 Bq). In practical terms, this means a few atoms per human body per year is the statistical limit. Therefore, any mendelevium must be handled with full heavy shielding, remote manipulators, and air filtration: exactly the same precautions as for plutonium or americium, but even more stringent because the allowed amount is virtually zero.

General safety notes: Mendelevium is chemically toxic as a heavy metal (like mercury or cadmium) if one were to swallow it, but the radiation hazard dominates by many orders of magnitude. Its alpha particles can damage tissues if inhaled or ingested. In open air, an alpha emitter of this mass would not penetrate skin, but it is still treated as a serious radiological hazard. Thus from a biological/environmental perspective, Md is one of the least benign elements imaginable: extremely dangerous in microgram doses and utterly foreign to any natural system.

History and Etymology

Mendelevium was first synthesized in 1955 at the University of California, Berkeley, by a team led by Albert Ghiorso and including Glenn Seaborg, Gregory Choppin, Bernard Harvey, and Stanley Thompson. They bombarded a target of Einsteinium-253 (only a billion atoms in total) with energetic alpha particles (helium nuclei) in the 60-inch cyclotron. This produced a few atoms of Mendelevium-256 at a time (each bombardment yielded about one new atom). To separate those atoms, Ghiorso’s team used a clever recoil technique: einsteinium was electroplated onto a thin gold foil, so that a newly created Md atom recoiled out of the target and was caught on a catcher foil behind it. The foils were then dissolved in acid and Md isolated via ion-exchange chemistry using α-hydroxyisobutyric acid as the eluant. The presence of Md was confirmed by detecting the characteristic spontaneous fission of its daughter, Fermium-256. In total only 17 atoms of Md were observed in the first experiments, making it the first element "discovered atom by atom." (This marked it as the first of the second hundred elements.)

Because elements beyond uranium were a major scientific prize, the new element was verified and submitted for naming in 1955. It was named mendelevium in honor of Dmitri Mendeleev, who laid the groundwork for the periodic table. A historical footnote: Seaborg had to secure permission from the U.S. government to name the element after a Russian scientist during the Cold War, and was granted it. Initially the symbol was proposed as "Mv", but in 1957 IUPAC changed it to Md as the official symbol.

The discovery of Md is notable not only for the chemistry but for the pioneering method. It proved that elements can be identified and studied even when produced one atom at a time – a milestone in heavy-element research. Ghiorso’s recoil-catcher method became a standard technique for discovering subsequent superheavy elements.

Table: Summary of Key Data for Mendelevium (Md).

Property	Value
Name	Mendelevium
Symbol	Md
Atomic number (Z)	101
Atomic mass	[258] u (no stable isotopes)
Electron configuration	[Rn] 5f¹³ 7s²
Common oxidation states	+3 (dominant), +2
Atomic radius	∼173 pm (covalent); Md³⁺ (coordination VI) ≈90 pm
Electronegativity (Pauling)	1.3
First ionization energy	6.58 eV
Phase at STP	Solid (predicted; metal)
Density (predicted)	~10–15 g/cm³
Melting point	~1100 K (827 °C; predicted)
Boiling point	Unknown (expected >1500 K)
Crystal structure	Hexagonal (predicted α-phase, P6₃/mmc; not measured)
Series (row/column)	Actinide (period 7, f-block)
Discovery	1955, Lawrence Berkeley Lab (Ghiorso et al.)
Named after	Dmitri Mendeleev (father of the periodic table)
Isotopes	A = 245–260 (all radioactive)
Longest-lived isotope	²⁵⁸Md (t₁/₂ ≈ 51.5 days)
CAS Registry Number	7440-11-1