101 Md Mendelevium 258*
Actinide f-block Period 7 Radioactive Synthetic

Mendelevium

Md · Element 101 · Actinide

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The first element produced and identified one atom at a time.

State at 20°C Solid~
Atomic mass 258 u
Electron configuration[Rn] 5f¹³ 7s²
100Fm All 118 102No

Structure

The mendelevium atom

Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.

Orbital cloud

Measured values

Property sheet

Every bar shows where mendelevium sits among all 118 elements for that property.

Physical

Density 10.3 g/cm³~ 67%
Melting point 1100 K~ 48%
Boiling point —
Specific heat —
Thermal conductivity —

Atomic

Atomic radius 246 pm 83%
Covalent radius 173 pm~
Van der Waals radius 246 pm

Electronic

Electronegativity 1.3 30%
Ionisation energy 634.9 kJ/mol 40%
Electron affinity —

Occurrence

Abundance in crust 0 mg/kg 0%

Identity

SymbolMd
Atomic number101
Atomic mass258 u
CategoryActinide
Blockf
Crystal structureunknown
Oxidation states+2, +3
Discovered1955
Discovered byGhiorso et al.

Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.

Same element, different neutrons

Isotopes of mendelevium

All mendelevium atoms have the same number of protons, 101, and differ only in their number of neutrons: those variants are its isotopes. None of them is stable. The longest-lived known isotope, Md-258, has 157 neutrons and a half-life of 51.50 days.

Long-lived and well-known isotopes of mendelevium
ISOTOPE Neutrons Natural Abundance Half-life Decay Mode
Md-256 155 — 77.7 min ε, α, SF
Md-257 156 — 5.52 hr ε, α, SF
Md-258 longest-lived 157 — 51.50 days α
Md-259 158 — 1.60 hr SF, α
Md-260 159 — 31.8 days SF (42%), α (25%), ε (23%)

Here the classroom shortcut, rounded atomic mass (258) minus atomic number, gives the same answer: 157.

Decay modes

ε
electron capture
α
alpha decay
SF
spontaneous fission

Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.

Size, to scale

How big is a mendelevium atom?

Radius 246 pm — that is 0.246 nm, so about 2.03 million of them side by side would span a millimetre.

Thermal range

Solid, liquid, gas — and when

Mendelevium melts at 1100 K; its boiling point has not been measured.

Where it sits

Position in the table

Mendelevium sits in the actinide series, printed below the main grid.

Other actinides

89Ac 90Th 91Pa 92U 93Np 94Pu 95Am 96Cm 97Bk 98Cf 99Es 100Fm 102No 103Lr
All actinides

The story

What mendelevium is, and how we found it

The first element produced and identified one atom at a time.

The discovery of mendelevium

Berkeley Laboratory Triumph (1955)

Mendelevium was discovered on February 19, 1955, by the team of Albert Ghiorso, Glenn T. Seaborg, Gregory Robert Choppin, Bernard G. Harvey, and Stanley G. Thompson at the University of California, Berkeley. This discovery marked a pivotal moment in the Cold War race to synthesize new elements.

Cold War Nuclear Connection

The discovery had dramatic origins in nuclear weapons testing. In 1952, the Ivy Mike thermonuclear test - a 10-megaton hydrogen bomb explosion - created enough neutron flux to produce previously unknown elements. The einsteinium-253 target for mendelevium, about a billion atoms, was made separately, by irradiating plutonium with neutrons in a reactor program begun in 1952.

One Atom at a Time

Using Berkeley's 60-inch cyclotron, the team bombarded just one billion einsteinium atoms with alpha particles - the first time in history a new element was produced and identified "one atom at a time." On that historic February morning, they detected five characteristic fission events that proved element 101's existence.

Nobel Legacy Naming

The element was named after Dmitri Mendeleev, creator of the periodic table, honoring his prediction that elements beyond uranium would be discovered. This naming represented both scientific tribute and Cold War diplomacy, acknowledging Russian scientific contributions during tense US-Soviet relations.

Scientific Significance

The discovery established new techniques for superheavy element research that remain fundamental to modern nuclear physics. The achievement showed that a new element could be made and identified one atom at a time.

Applications

What mendelevium is used for

Nuclear Physics Research

Mendelevium serves as a crucial stepping stone in superheavy element synthesis research, providing insights into nuclear shell structure and stability. Scientists use Md isotopes to understand the magic numbers that govern nuclear stability, particularly around the predicted "island of stability" for superheavy elements.

Advanced Nuclear Studies

Research teams employ mendelevium in nuclear fission studies to investigate spontaneous fission processes and alpha decay chains. The element's relatively long half-life (77.7 minutes for 256Md) makes it invaluable for studying transuranium element chemistry and physics properties.

Chemical Property Investigation

Mendelevium enables new research into superheavy element chemistry, allowing scientists to study oxidation states, complex formation, and chemical behavior of elements beyond the actinide series. These studies help validate theoretical predictions about electron configuration and chemical bonding in unusual atomic environments.

Theoretical Model Validation

Physicists use mendelevium data to refine nuclear models and calculations that predict the existence and properties of even heavier elements. This research drives the development of advanced particle accelerators and detection systems for future superheavy element discoveries.

Future Space Applications

While theoretical, understanding mendelevium's properties contributes to research on exotic matter that could potentially transform space propulsion systems and energy generation technologies in the distant future.

Research Laboratory Applications

Mendelevium's primary applications remain confined to specialized nuclear research facilities worldwide. Major laboratories including Berkeley Lab, GSI Helmholtz Centre, and RIKEN use mendelevium isotopes for fundamental nuclear physics research and superheavy element synthesis studies.

Nuclear Structure Studies

Research teams utilize mendelevium in nuclear spectroscopy experiments to measure gamma-ray emissions, alpha decay energies, and fission fragment distributions. These measurements provide critical data for understanding nuclear shell effects and predicting properties of undiscovered superheavy elements.

Theoretical Physics Validation

Mendelevium serves as a benchmark element for testing quantum mechanical calculations and nuclear models. Scientists compare experimental data from mendelevium isotopes with theoretical predictions to refine their understanding of nuclear forces and electron behavior in superheavy atoms.

Advanced Detection Technology

The study of mendelevium drives innovation in ultra-sensitive detection systems, including time-of-flight mass spectrometry, alpha-particle detectors, and sophisticated particle identification systems that advance our ability to study extremely rare nuclear processes.

Where it comes from

Natural occurrence

0 mg/kg of Earth's crust · more abundant than 0% of elements

Synthetic Origin Only

Mendelevium has never been found in nature, on Earth or elsewhere. This superheavy element has only ever been made through artificial nuclear synthesis in sophisticated particle accelerators, making it one of the rarest substances ever produced by humanity.

Laboratory Production Process

Scientists create mendelevium by bombarding einsteinium-253 targets with alpha particles (helium-4 nuclei) in cyclotrons or linear accelerators. The Berkeley team's original 1955 synthesis used just one billion einsteinium atoms, producing mendelevium literally "one atom at a time" - a remarkable achievement in nuclear physics.

Rarity and Instability

The most stable mendelevium isotope, 258Md, has a half-life of only 51.5 days, meaning any mendelevium atoms that might have existed during Earth's formation would have decayed billions of years ago. Current production yields are measured in individual atoms or small clusters.

Global Production Limitations

Worldwide production of mendelevium is estimated at fewer than a few thousand atoms per year across all research facilities combined. The element's synthesis requires rare einsteinium targets, advanced particle accelerators, and enormous amounts of energy, making it astronomically expensive to produce.

Cosmic Absence

Unlike lighter elements formed in stellar nucleosynthesis, no mendelevium survives in nature because of its short half-lives, although the r-process in neutron star mergers is thought to make even heavier nuclei that briefly pass through this region. Every known mendelevium atom was made in a laboratory.

Handling

Safety

Mendelevium is radioactive. It has no stable isotope — every nucleus decays. Handling requires appropriate shielding and licensing.

Radiation Hazard

Mendelevium is radioactive, but it is made only in extremely small amounts in accelerators, mainly by bombarding einsteinium targets with alpha particles. Its longest-lived isotope, mendelevium-258, has a half-life of about 51 days. No weighable amount of mendelevium has ever existed, so the atoms themselves cannot deliver a meaningful radiation dose to anyone; their alpha particles cannot penetrate the outer layer of skin and could harm tissue only from inside the body.

Where the Real Hazards Are

The hazards in this research come from the experiment, not the element: the accelerator beam, the radioactive einsteinium targets and activated equipment, and the waste these produce. Contamination stays confined to the laboratory, and the waste is managed as radioactive waste under national radiation protection rules.

Worker Protection

Staff follow the standard occupational radiation rules: annual dose limits, personal dose monitoring and keeping doses as low as reasonably achievable. Workers under 18 face much lower dose limits than adults: one tenth of the adult limits under US Nuclear Regulatory Commission rules (10 CFR 20.1207), and 1 mSv a year at US Department of Energy laboratories (10 CFR 835.207). A worker who declares a pregnancy is limited to 5 mSv to the fetus over the whole pregnancy (10 CFR 20.1208 and 835.206).

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Quick answers

Mendelevium: common questions

What is mendelevium?

Mendelevium (symbol Md) is element 101 on the periodic table, an actinide in period 7. The first element produced and identified one atom at a time. At room temperature it is expected to be a solid, and it is radioactive.

What is the electron configuration of mendelevium?

Mendelevium's ground-state electron configuration is [Rn] 5f¹³ 7s², giving 7 occupied shells with the distribution 2, 8, 18, 32, 31, 8, 2.

What are the melting and boiling points of mendelevium?

No sample of mendelevium has ever been condensed, so its melting point is calculated rather than measured: it is predicted to melt at 1100 K (826.9 °C). Its boiling point has not been measured.

What is the atomic mass of mendelevium?

Mendelevium has no stable isotope, so it has no standard atomic weight. The figure quoted, 258, is the mass number of its longest-lived known isotope.

How many neutrons does mendelevium have?

Mendelevium has no stable isotope. Its longest-lived known isotope, Md-258, has 157 neutrons (mass number 258 minus atomic number 101) and a half-life of 51.50 days. The classroom shortcut, rounded atomic mass (258) minus atomic number, gives the same 157.

How dense is mendelevium?

Mendelevium is predicted to have a density of about 10.3 g/cm³, though no sample large enough to weigh has ever been made. Water is 1.0 g/cm³ for comparison.

What is the electronegativity of mendelevium?

Mendelevium has a Pauling electronegativity of 1.3. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.

Who discovered mendelevium, and when?

Mendelevium was discovered in 1955 by Ghiorso et al.. Origin of the name: Dmitri Mendeleev.

How common is mendelevium on Earth?

Mendelevium does not occur naturally on Earth in any meaningful quantity — it is made in a reactor or an accelerator.

Is mendelevium radioactive?

Yes. Mendelevium has no stable isotope — every one of its nuclei decays. It does not occur in usable quantities in nature and must be synthesised.

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