Meitnerium (pronounced /maɪtˈnɪəriəm/ myet-NEER-ee-əm or /maɪtˈnɜriəm/ myet-NER-ee-əm) is a chemical element with the symbol Mt and atomic number 109. It is placed as the heaviest member of group 9 (or VIII) in the periodic table but a sufficiently stable isotope is not known at this time which would allow chemical experiments to confirm its position, unlike its lighter neighbours.
It was first synthesized in 1982 and several isotopes are currently known. The heaviest and the most stable isotope known is Mt-278, with a half-life of ~8 s.
Meitnerium was first synthesized on August 29, 1982 by a German research team led by Peter Armbruster and Gottfried Münzenberg at the Institute for Heavy Ion Research (Gesellschaft für Schwerionenforschung) in Darmstadt. The team bombarded a target of bismuth-209 with accelerated nuclei of iron-58 and detected a single atom of the isotope meitnerium-266:
Element 109 was formerly known as Unnilennium, bearing the symbol Une.
Historically, element 109 has been referred to as eka-iridium.
The name meitnerium (Mt) was suggested in honor of the Austrian physicist Lise Meitner. In 1997, the name was officially adopted by the IUPAC.
The team at RIKEN, Japan, have indicated that as part of their ongoing studies using 248Cm targets, they may study the new reaction 248Cm(27Al,xn) in the future.
Isotopes and nuclear properties
Target-Projectile Combinations leading to Z=109 compound nuclei
The below table contains various combinations of targets and projectiles which could be used to form compound nuclei with Z=109.
This section deals with the synthesis of nuclei of meitnerium by so-called "cold" fusion reactions. These are processes which create compound nuclei at low excitation energy (~10-20 MeV, hence "cold"), leading to a higher probability of survival from fission. The excited nucleus then decays to the ground state via the emission of one or two neutrons only.
The first success in this reaction was in 1982 by the GSI team in their discovery experiment with the identification of a single atom of 266Mt in the 1n neutron evaporation channel. The GSI team used the parent-daughter correlation technique. After an initial failure in 1983, in 1985 the team at the FLNR, Dubna, observed alpha decays from the descendant 246Cf indicating the formation of meitnerium. The GSI synthesised a further 2 atoms of 266Mt in 1988 and continued in 1997 with the detection of 12 atoms during the measurement of the 1n excitation function.  
This reaction was first studied in 1985 by the team in Dubna. They were able to detect the alpha decay of the descendant 246Cf nuclei indicating the formation of meitnerium atoms. In 2007, in a continuation of their study of the effect of odd-Z projectiles on yields of evaporation residues in cold fusion reactions, the team at LBNL synthesised 266Mt and were able to correlate the decay with known daughters.
There are indications that this cold fusion reaction using a tantalum target was attempted in August 2001 at the GSI. No details can be found suggesting that no atoms of meitnerium were detected.
In 2002-2003, the team at LBNL attempted the above reaction in order to search for the isotope 271Mt with hope that it may be sufficiently stable to allow a first study of the chemical properties of meitnerium. Unfortunately, no atoms were detected and a cross section limit of 1.5 pb was measured for the 4n channel at the projectile energy used. 
Attempts to produce long-living isotopes of meitnerium were first performed by Ken Hulet at the Lawrence Livermore National Laboratory (LLNL) in 1988 using the asymmetric hot fusion reaction above. They were unable to detect any product atoms and established a cross section limit of 1 nb.
As a decay product
Isotopes of meitnerium have also been detected in the decay of heavier elements. Observations to date are shown in the table below:
Chronology of isotope discovery
Two atoms of 270Mt have been identified in the decay chains of 278113. The two decays have very different lifetimes and decay energies and are also produced from two apparently different isomers in 274Rg. The first isomer decays by emission of an 10.03 MeV alpha particle with a lifetime 7.2 ms. The other decays by emitting an alpha particle with a lifetime of 1.63 s. An assignment to specific levels is not possible with the limited data available. Further research is required.
The alpha decay spectrum for 268Mt appears to be complicated from the results of several experiments. Alpha lines of 10.28,10.22 and 10.10 MeV have been observed. Half-lives of 42 ms, 21 ms and 102 ms have been determined. The long-lived decay is associated with alpha particles of energy 10.10 MeV and must be assigned to an isomeric level. The discrepancy between the other two half-lives has yet to be resolved. An assignment to specific levels is not possible with the data available and further research is required.
Chemical yields of isotopes
The table below provides cross-sections and excitation energies for cold fusion reactions producing meitnerium isotopes directly. Data in bold represent maxima derived from excitation function measurements. + represents an observed exit channel.
Evaporation residue cross sections
The below table contains various targets-projectile combinations for which calculations have provided estimates for cross section yields from various neutron evaporation channels. The channel with the highest expected yield is given.
HIVAP = heavy-ion vaporisation statistical-evaporation model; σ = cross section
Extrapolated chemical properties
Mt should be a very heavy metal with a density around 30 g/cm3 (Co: 8.9, Rh: 12.5, Ir: 22.5) and a high melting point around 2600-2900°C (Co: 1480, Rh: 1966, Ir: 2454). It should be very corrosion-resistant; even more so than Ir which is currently the most corrosion-resistant metal known.
Meitnerium is projected to be the sixth member of the 6d series of transition metals and the heaviest member of group 9 in the Periodic Table, below cobalt, rhodium and iridium. This group of transition metals is the first to show lower oxidation states and the +9 state is not known. The latter two members of the group show a maximum oxidation state of +6, whilst the most stable states are +4 and +3 for iridium and +3 for rhodium. Meitnerium is therefore expected to form a stable +3 state but may also portray stable +4 and +6 states.
The +VI state in group 9 is known only for the fluorides which are formed by direct reaction. Therefore, meitnerium should form a hexafluoride, MtF6. This fluoride is expected to be more stable than iridium(VI) fluoride, as the +6 state becomes more stable as the group is descended.
In combination with oxygen, rhodium forms Rh2O3 whilst iridium is oxidised to the +4 state in IrO2. Meitnerium may therefore show a dioxide, MtO2, if eka-iridium reactivity is shown.
The +3 state in group 9 is common in the trihalides (except fluorides) formed by direct reaction with halogens. Meitnerium should therefore form MtCl3, MtBr3 and MtI3 in an analogous manner to iridium.
1. ^ a b c Münzenberg, G. (1982). "Observation of one correlated α-decay in the reaction 58Fe on 209Bi→267109". Zeitschrift für Physik a Atoms and Nuclei 309: 89. doi:10.1007/BF01420157.
* WebElements.com: Meitnerium