Unknown propertiesElement 109 · d-block

Meitnerium (Mt)

Meitnerium (Mt), element 109, is a synthetic superheavy element whose chemical properties have not been measured; its most stable known isotope has mass number 278. Only a few atoms have ever been made, so its bulk properties are predictions.

109MtMeitnerium[278]

Atomic number

109

protons

Atomic weight

[278]

most stable isotope

Melting point

—

Boiling point

—

Density

37.4 g/cm³

predicted

Electronegativity

—

Pauling scale

State at 25 °C

Unknown

predicted solid

Discovered

1982

Germany (Darmstadt)

Meitnerium at a glance

Identity

Symbol	Mt
Atomic number (Z)	109
Atomic weight	[278] mass number of the most stable isotope
Category	Unknown properties
Group	9
Period	7
Block	d-block
CAS number	54038-01-6

Atomic properties

Electron configuration	[Rn] 5f¹⁴ 6d⁷ 7s² Full: 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
Covalent radius	129 pm

Physical properties

State at 25 °C	Unknown (predicted solid)
Density	37.4 g/cm³ (predicted)

Isotopes & abundance

Natural occurrence	Synthetic
Radioactive	Yes
Most stable isotope	²⁷⁸Mt — half-life 6 s
Known isotopes	18

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Meitnerium electron configuration

Meitnerium has 109 electrons. Written in short form, its ground-state configuration is [Rn] 5f¹⁴ 6d⁷ 7s² — the electrons of the noble gas radon, plus 5f¹⁴ 6d⁷ 7s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹⁴ 6s² 6p⁶ 6d⁷ 7s².

5f↑↓↑↓↑↓↑↓↑↓↑↓↑↓

6d↑↓↑↓↑↑↑

7s↑↓

Discovery and name

Discovered

1982 — Peter Armbruster and Gottfried Münzenberg (GSI) · Germany (Darmstadt).

Name

Lise Meitner.

Known for

Only a handful of atoms have been made; its chemical properties have not been measured.

Isotopes of meitnerium

Meitnerium has no stable isotopes and does not occur naturally in significant amounts. Its longest-lived known isotope is:

Isotope	Protons	Neutrons	Half-life
²⁷⁸Mt	109	169	6 s

18 isotopes of meitnerium are known in our data set (half-lives as published in nuclear data tables).

Meitnerium compared with group 9

The other elements in group 9, from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Cobalt	27	58.933	1,495 °C	2,927 °C	8.86 g/cm³	1.88
Rhodium	45	102.91	1,963 °C	3,695 °C	12.4 g/cm³	2.28
Iridium	77	192.22	2,446 °C	4,428 °C	22.562 g/cm³	2.2
Meitnerium	109	[278]	—	—	37.4 g/cm³ (pred.)	—

Frequently asked questions

4 answers

What is the atomic mass of meitnerium?

Meitnerium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [278], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does meitnerium have?

A neutral meitnerium atom has 109 protons and 109 electrons. Its most stable isotope, meitnerium-278, has 169 neutrons.

What is the electron configuration of meitnerium?

[Rn] 5f¹⁴ 6d⁷ 7s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹⁴ 6s² 6p⁶ 6d⁷ 7s²).

Who discovered meitnerium?

1982 — Peter Armbruster and Gottfried Münzenberg (GSI) (Germany (Darmstadt)).