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Mt

Meitnerium

properties predicted

Meitnerium (Mt) is a transition metal with atomic number 109. It has no stable isotope and no characteristic isotopic composition on Earth, so it has no standard atomic weight. CIAAW lists meitnerium-277 and meitnerium-278 as its longest-lived isotopes, and the mass number used here is 278. Its electron configuration is predicted to be [Rn] 5f14 6d7 7s2.

Meitnerium key data

Symbol Mt
Atomic number 109
Mass number (one of its longest-lived isotopes) 278
Protons 109
Neutrons (meitnerium-278) 169
Electrons (neutral atom) 109
Category Transition metal
Period 7
Group 9
Block d-block
Noble gas notation [Rn] 5f14 6d7 7s2
Full electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f14 6s2 6p6 6d7 7s2
Electrons per shell 2, 8, 18, 32, 32, 15, 2

Mass number: IUPAC CIAAW radioactive elements, from NUBASE2020.

The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.

Properties of meitnerium

From PubChem, the US National Institutes of Health. Public domain. Most properties of this element are predicted, because only a few atoms have ever been made.

Predicted state at room temperature Solid
Predicted oxidation states +9, +8, +6, +4, +3, +1
Discovered 1982

Meitnerium electron configuration

The full electron configuration of meitnerium is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f14 6s2 6p6 6d7 7s2. With the noble-gas core collapsed it is [Rn] 5f14 6d7 7s2, and the outermost subshell is 7s2.

Bohr model of meitnerium (Mt): a nucleus and seven shells holding 2, 8, 18, 32, 32, 15 and 2 electrons from the inside out.
Bohr model of meitnerium (Mt). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.
Orbital box diagram of meitnerium (Mt), [Rn] 5f14 6d7 7s2, with the radon core drawn as one bar: one box per orbital, filled by Hund’s rule, showing three unpaired electrons, in 6d.
Orbital diagram of meitnerium (Mt). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one meitnerium atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

  1. one atom = 278 / (6.02214076 × 10²³)
  2. = 4.6163 × 10⁻²² g

Atoms in one gram

  1. atoms per gram = 6.02214076 × 10²³ / 278
  2. = 2.1662 × 10²¹ atoms

Meitnerium has no standard atomic weight, because it has neither a stable isotope nor a characteristic isotopic composition on Earth. The figure used above is the mass number of meitnerium-278, so treat these as nominal.

Meitnerium on the periodic table

Meitnerium is in period 7, group 9, in the d-block.

The other elements of group 9 are cobalt, rhodium and iridium.

The rest of period 7

Work with meitnerium