ActinideElement 101 · f-block

Mendelevium (Md)

Mendelevium (Md) is an actinide with atomic number 101; its most stable known isotope has mass number 258. At room temperature it is a solid that melts at 827 °C (estimated).

101MdMendelevium[258]

Atomic number

101

protons

Atomic weight

[258]

most stable isotope

Melting point

827 °C

estimated

Boiling point

—

Density

10.3 g/cm³

predicted

Electronegativity

1.3

Pauling scale

State at 25 °C

Solid

Discovered

1955

United States (Berkeley)

Mendelevium at a glance

Identity

Symbol	Md
Atomic number (Z)	101
Atomic weight	[258] mass number of the most stable isotope
Category	Actinide
Group	— (f-block: actinide series)
Period	7
Block	f-block
CAS number	7440-11-1

Atomic properties

Electron configuration	[Rn] 5f¹³ 7s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹³ 6s² 6p⁶ 7s²
Electrons per shell	2, 8, 18, 32, 31, 8, 2
Oxidation states	+3 Less common: +2
Electronegativity (Pauling)	1.3
First ionisation energy	6.58 eV (634.9 kJ/mol) 2nd: 12.4 eV · 3rd: 24.3 eV
Covalent radius	173 pm
Van der Waals radius	246 pm

Physical properties

State at 25 °C	Solid
Melting point	827 °C · 1,520.6 °F · 1,100.15 K estimated
Density	10.3 g/cm³ (predicted)

Isotopes & abundance

Natural occurrence	Synthetic
Radioactive	Yes
Most stable isotope	²⁵⁸Md — half-life 51.59 days
Known isotopes	19

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

Mendelevium electron configuration

Mendelevium has 101 electrons. Written in short form, its ground-state configuration is [Rn] 5f¹³ 7s² — the electrons of the noble gas radon, plus 5f¹³ 7s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹³ 6s² 6p⁶ 7s².

5f↑↓↑↓↑↓↑↓↑↓↑↓↑

7s↑↓

Discovery and name

Discovered

1955 — Albert Ghiorso, Bernard Harvey, Gregory Choppin, Stanley Thompson and Glenn T. Seaborg · United States (Berkeley).

Name

Dmitri Mendeleev, creator of the periodic table.

Known for

The first element synthesised one atom at a time — the first experiment produced just 17 atoms.

Isotopes of mendelevium

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

Isotope	Protons	Neutrons	Half-life
²⁵⁸Md	101	157	51.59 days

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

Mendelevium and its neighbours in the actinide series

Neighbouring elements in the same f-block series:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Californium	98	[251]	900 °C	—	15.1 g/cm³	1.3
Einsteinium	99	[252]	860 °C (est.)	—	8.84 g/cm³ (pred.)	1.3
Fermium	100	[257]	1,527 °C (est.)	—	9.7 g/cm³ (pred.)	1.3
Mendelevium	101	[258]	827 °C (est.)	—	10.3 g/cm³ (pred.)	1.3
Nobelium	102	[259]	827 °C (est.)	—	9.9 g/cm³ (pred.)	1.3
Lawrencium	103	[266]	1,627 °C (est.)	—	15.6 g/cm³ (pred.)	1.3

Frequently asked questions

6 answers

What is the atomic mass of mendelevium?

Mendelevium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [258], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does mendelevium have?

A neutral mendelevium atom has 101 protons and 101 electrons. Its most stable isotope, mendelevium-258, has 157 neutrons.

What is the electron configuration of mendelevium?

[Rn] 5f¹³ 7s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹³ 6s² 6p⁶ 7s²).

What are the melting and boiling points of mendelevium?

Mendelevium melts at 827 °C (1,520.6 °F) (estimated).

Is mendelevium a metal?

Mendelevium is a metal. It is in the f-block, period 7, and is a solid at room temperature.

Who discovered mendelevium?

1955 — Albert Ghiorso, Bernard Harvey, Gregory Choppin, Stanley Thompson and Glenn T. Seaborg (United States (Berkeley)).