Unknown propertiesElement 111 · d-block

Roentgenium (Rg)

Roentgenium (Rg), element 111, is a synthetic superheavy element whose chemical properties have not been measured; its most stable known isotope has mass number 282. Only a few atoms have ever been made, so its bulk properties are predictions.

111RgRoentgenium[282]

Atomic number

111

protons

Atomic weight

[282]

most stable isotope

Melting point

—

Boiling point

—

Density

28.7 g/cm³

predicted

Electronegativity

—

Pauling scale

State at 25 °C

Unknown

predicted solid

Discovered

1994

Germany (Darmstadt)

Roentgenium at a glance

Identity

Symbol	Rg
Atomic number (Z)	111
Atomic weight	[282] mass number of the most stable isotope
Category	Unknown properties
Group	11
Period	7
Block	d-block
CAS number	54386-24-2

Atomic properties

Electron configuration	[Rn] 5f¹⁴ 6d¹⁰ 7s¹ Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹⁴ 6s² 6p⁶ 6d¹⁰ 7s¹
Electrons per shell	2, 8, 18, 32, 32, 18, 1
Covalent radius	121 pm

Physical properties

State at 25 °C	Unknown (predicted solid)
Density	28.7 g/cm³ (predicted)

Isotopes & abundance

Natural occurrence	Synthetic
Radioactive	Yes
Most stable isotope	²⁸²Rg — half-life 130 s
Known isotopes	15

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

Roentgenium electron configuration

Roentgenium has 111 electrons. Written in short form, its ground-state configuration is [Rn] 5f¹⁴ 6d¹⁰ 7s¹ — the electrons of the noble gas radon, plus 5f¹⁴ 6d¹⁰ 7s¹.Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹⁴ 6s² 6p⁶ 6d¹⁰ 7s¹.

An exception to the filling rule. The simple aufbau (Madelung) order predicts 7s², 6d⁹, but roentgenium actually has 7s¹, 6d¹⁰ (calculated — it hasn't been measured). Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.

5f↑↓↑↓↑↓↑↓↑↓↑↓↑↓

6d↑↓↑↓↑↓↑↓↑↓

7s↑

Discovery and name

Discovered

1994 — Sigurd Hofmann and co-workers (GSI) · Germany (Darmstadt).

Name

Wilhelm Conrad Röntgen, discoverer of X-rays.

Known for

Expected to be a noble metal like gold; only a few atoms have been made.

Isotopes of roentgenium

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

Isotope	Protons	Neutrons	Half-life
²⁸²Rg	111	171	130 s

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

Roentgenium compared with group 11

The other elements in group 11, from top to bottom of the table:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Copper	29	63.546	1,085 °C	2,560 °C	8.96 g/cm³	1.9
Silver	47	107.87	962 °C	2,162 °C	10.5 g/cm³	1.93
Gold	79	196.97	1,064 °C	2,836 °C	19.3 g/cm³	2.54
Roentgenium	111	[282]	—	—	28.7 g/cm³ (pred.)	—

Frequently asked questions

4 answers

What is the atomic mass of roentgenium?

Roentgenium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [282], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does roentgenium have?

A neutral roentgenium atom has 111 protons and 111 electrons. Its most stable isotope, roentgenium-282, has 171 neutrons.

What is the electron configuration of roentgenium?

[Rn] 5f¹⁴ 6d¹⁰ 7s¹ (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹⁴ 6s² 6p⁶ 6d¹⁰ 7s¹).

Who discovered roentgenium?

1994 — Sigurd Hofmann and co-workers (GSI) (Germany (Darmstadt)).