Meitnerium (₁₀₉Mt) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was ²⁶⁶Mt in 1982, and this is also the only isotope directly synthesized; all other isotopes are only known as decay products of heavier elements. There are eight known isotopes, from ²⁶⁶Mt to ²⁷⁸Mt. There may also be two isomers. The longest-lived of the known isotopes is ²⁷⁸Mt with a half-life of 8 seconds. The unconfirmed heavier ²⁸²Mt appears to have an even longer half-life of 67 seconds.

Super-heavy elements such as meitnerium are produced by bombarding lighter elements in particle accelerators that induce fusion reactions. Whereas the lightest isotope of meitnerium, meitnerium-266, can be synthesized directly this way, all the heavier meitnerium isotopes have only been observed as decay products of elements with higher atomic numbers.^[5]

Depending on the energies involved, the former are separated into "hot" and "cold". In hot fusion reactions, very light, high-energy projectiles are accelerated toward very heavy targets (actinides), giving rise to compound nuclei at high excitation energy (~40–50 MeV) that may either fission or evaporate several (3 to 5) neutrons.^[6] In cold fusion reactions, the produced fused nuclei have a relatively low excitation energy (~10–20 MeV), which decreases the probability that these products will undergo fission reactions. As the fused nuclei cool to the ground state, they require emission of only one or two neutrons, and thus, allows for the generation of more neutron-rich products.^[5] Nevertheless, the products of hot fusion tend to still have more neutrons overall. The latter is a distinct concept from that of where nuclear fusion claimed to be achieved at room temperature conditions (see cold fusion).^[7]

The table below contains various combinations of targets and projectiles which could be used to form compound nuclei with Z = 109.

After the first successful synthesis of meitnerium in 1982 by the GSI team,^[8] a team at the Joint Institute for Nuclear Research in Dubna, Russia, also tried to observe the new element by bombarding bismuth-209 with iron-58. In 1985 they managed to identity alpha decays from the descendant isotope ²⁴⁶Cf indicating the formation of meitnerium. The observation of a further two atoms of ²⁶⁶Mt from the same reaction was reported in 1988 and of another 12 in 1997 by the German team at GSI.^[9][10]

The same meitnerium isotope was also observed by the Russian team at Dubna in 1985 from the reaction:

²⁰⁸
₈₂Pb
+ ⁵⁹
₂₇Co
→ ²⁶⁶
₁₀₉Mt
+
n

by detecting the alpha decay of the descendant ²⁴⁶Cf nuclei. In 2007, an American team at the Lawrence Berkeley National Laboratory (LBNL) confirmed the decay chain of the ²⁶⁶Mt isotope from this reaction.^[11]

In 2002–2003, the team at LBNL attempted to generate the isotope ²⁷¹Mt to study its chemical properties by bombarding uranium-238 with chlorine-37, but without success.^[12] Another possible reaction that would form this isotope would be the fusion of berkelium-249 with magnesium-26; however, the yield for this reaction is expected to be very low due to the high radioactivity of the berkelium-249 target.^[13] Other potentially longer-lived isotopes were unsuccessfully targeted by a team at Lawrence Livermore National Laboratory (LLNL) in 1988 by bombarding einsteinium-254 with neon-22.^[12]

All the isotopes of meitnerium except meitnerium-266 have been detected only in the decay chains of elements with a higher atomic number, such as roentgenium. Roentgenium currently has eight known isotopes; all but one of them undergo alpha decays to become meitnerium nuclei, with mass numbers between 268 and 282. Parent roentgenium nuclei can be themselves decay products of nihonium, flerovium, moscovium, livermorium, or tennessine.^[18] For example, in January 2010, the Dubna team (JINR) identified meitnerium-278 as a product in the decay of tennessine via an alpha decay sequence:^[14]

²⁹⁴
₁₁₇Ts
→ ²⁹⁰
₁₁₅Mc
+ ⁴
₂He

²⁹⁰
₁₁₅Mc
→ ²⁸⁶
₁₁₃Nh
+ ⁴
₂He

²⁸⁶
₁₁₃Nh
→ ²⁸²
₁₁₁Rg
+ ⁴
₂He

²⁸²
₁₁₁Rg
→ ²⁷⁸
₁₀₉Mt
+ ⁴
₂He

²⁷⁰Mt

Two atoms of ²⁷⁰Mt have been identified in the decay chains of ²⁷⁸Nh. The two decays have very different lifetimes and decay energies and are also produced from two apparently different isomers of ²⁷⁴Rg. The first isomer decays by emission of an alpha particle with energy 10.03 MeV and has a lifetime of 7.16 ms. The other alpha decays with a lifetime of 1.63 s; the decay energy was not measured. An assignment to specific levels is not possible with the limited data available and further research is required.^[16]

²⁶⁸Mt

The alpha decay spectrum for ²⁶⁸Mt appears to be complicated from the results of several experiments. Alpha particles of energies 10.28, 10.22 and 10.10 MeV have been observed, emitted from ²⁶⁸Mt atoms with half-lives of 42 ms, 21 ms and 102 ms respectively. The long-lived decay 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.^[17]

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.

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.

DNS = Di-nuclear system; HIVAP = heavy-ion vaporisation statistical-evaporation model; σ = cross section

• Isotope masses from:
  • M. Wang; G. Audi; A. H. Wapstra; F. G. Kondev; M. MacCormick; X. Xu; et al. (2012). "The AME2012 atomic mass evaluation (II). Tables, graphs and references" (PDF). Chinese Physics C. 36 (12): 1603–2014. Bibcode:2012ChPhC..36....3M. doi:10.1088/1674-1137/36/12/003. hdl:11858/00-001M-0000-0010-23E8-5. S2CID 250839471.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.
  • GSI (2011). "Superheavy Element Research at GSI" (PDF). GSI. Retrieved 7 February 2013.