Mendelevium was created by directing helium nuclei at einsteinium in an accelerator. Detecting the resulting radioactive atoms established a new element beyond fermium. It remains a research subject, connecting the familiar ordering of the periodic table to experiments that assemble and identify heavier nuclei.
FROM ELEMENT TO APPLICATION
AN ELEMENT IN CONTEXT
What evidence distinguishes a reported new mendelevium isotope from a new element?
Mendelevium-244 in isotope-discovery research
FOLLOW THIS RELATIONSHIP
Keep proton identity fixed
Mendelevium-244 is a proposed isotope assignment within element 101. A different mass number changes the nuclear inventory without making the nucleus a different chemical element.
Explore the model (optional)
MdFORM → INTERACTION → CONSEQUENCE
Follow the explanation in either direction. These are authored steps, not a physical clock.
Optional: check your understanding
USE THE EXPLANATION
What would the reported identification add to the periodic table?
Atomic identity, reference properties and structure models
INTERACTIVE 3D101 visible sites
An electron censusDrag to rotate · scroll or pinch to zoom
[Rn] 5f13 7s2. Grouping electrons by principal shell connects the neutral-atom configuration to periodic-table patterns.
What this representation means
A three-dimensional counting diagram. Dots are count markers, not measured electron positions; spherical placement and spacing are arbitrary. There are no electron trajectories here. Orbitals describe quantum states.
Mendelevium was discovered at Berkeley in 1955 using an einsteinium target and methods able to identify exceptionally small numbers of atoms. Naming it for Dmitri Mendeleev honored the periodic system that helped organize the search. The element did not arrive as a bottle of new material. Its discovery demonstrated that carefully connected nuclear and chemical observations could establish identity at a scale far below conventional laboratory chemistry.
1955
Element discovered
The Berkeley team identifies mendelevium using an einsteinium target.
2020
New isotope reported
Berkeley researchers report mendelevium-244, extending the element’s isotope studies.
Mendelevium is produced artificially for research, with no known natural deposit or ordinary commercial supply. Experiments obtain small numbers of atoms that must be identified amid other nuclear products. An discovered decades after the element adds a new nuclear variant, not another entry to the periodic table.
Mendelevium’s chemistry includes +3 and +2 states, making it useful for exploring how late actinides depart from a simple all-trivalent pattern. Its neutral configuration has a nearly filled 5f shell. These electronic questions differ from the radioactive properties that allow researchers to detect and identify individual isotopes.
Each section links its supporting references. Descriptions and diagrams were authored for this atlas. Source checks completed. Source checks dated 2026-09-06.
CITE THIS PAGE
Family labels and the group-3 layout are teaching conventions. A table position does not establish an unknown bulk property. Electronic configurations refer to neutral atoms; ions and material environments require their own account.
Read each reference property together with its units, conditions and the reference information below.
Not yet recorded: electron affinity, second ionization energy, ground-state term symbol, van der waals radius, thermal conductivity, specific heat capacity, electrical resistivity, crystal structure, lattice parameter a, space group, abundance in seawater, abundance in the universe, annual production, supply-risk listing, hazard classification, biological role, cas registry number, pubchem cid, wikidata item. These families are declared and awaiting an ingestion run against the source each names; they are listed here rather than shown as empty rows.
Physical
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Phase at 20 °C
Not recorded
293.15 K, 101.325 kPa
Melting point
Not recorded
at standard pressure unless noted
Boiling point
Not recorded
101.325 kPa
Density
Not recorded
solids and liquids at 293 K; gases at 273 K and 101.325 kPa
Atomic and electronic
Intended reference: Pauling electronegativity scale · Widely tabulated; original in The Nature of the Chemical Bond
Electronegativity in Pauling
1.3 Pauling
Pauling scale
First ionization energy in kJ/mol
635 kJ/mol
gas-phase neutral atom, ground state
Covalent radius
Not recorded
single-bond covalent radius
Common oxidation states
+3
states met in ordinary compounds; not exhaustive
Occurrence and identity
Intended reference: CRC Handbook of Chemistry and Physics · CRC Press
Abundance in the crust
Not recorded
upper continental crust, mass fraction
Has stable isotopes
Noderived
Discovery year in CE
1955 CEderived
WHERE IT SITS AMONG ITS NEIGHBORS
Electronegativity across period 7
Md 1.3 Pauling · Mendelevium
Electronegativity across period 7. Exact values.
Ra
0.9
Ac
1.1
Th
1.3
Pa
1.5
U
1.38
Np
1.36
Pu
1.28
Am
1.13
Cm
1.28
Bk
1.3
Cf
1.3
Es
1.3
Fm
1.3
Md
1.3
No
1.3
Lr
1.3
The trend uses the same reference-property values as the periodic table. Elements with no recorded value are left out of the line rather than drawn at zero.
