ELEMENTLE

Element 99 · Actinide

Einsteinium

Period 7

Einsteinium is a synthetic element with symbol Es and atomic number 99. It is the seventh transuranic element, and an actinide. Einsteinium was discovered as a component of the debris of the first hydrogen bomb explosion in 1952, and named after Albert Einstein.

Three reasons Einsteinium stands out

  1. 1

    Synthetic element, named after famous theoretical physicist.

  2. 2

    It was secretly discovered in the radioactive debris of the first hydrogen bomb test in 1952.

  3. 3

    So little of it has ever been made that studying its basic chemical properties remains a significant scientific challenge.

Discovered in the ashes of a hydrogen bomb

Einsteinium was identified in debris from the 1952 Ivy Mike thermonuclear test. Its discovery was initially classified, and only minuscule quantities have ever been isolated. Intense self-irradiation damages samples and produces heat, complicating attempts to measure what ordinary einsteinium chemistry would look like.

Einsteinium at a glance
Atomic number / protons99
SymbolEs
Family (Elementle)Actinide
Classification (PubChem)Actinide
Period7
GroupLanthanide / actinide series
Atomic mass (u)252.0830
Standard stateSolid
Electron configuration[Rn]7s2 5f11
Reported oxidation states+3
Electronegativity (Pauling)1.3
Atomic radius (pm)245
First ionization energy (eV)6.42
Electron affinity (eV)Not available
Density (g/cm³)Not available
Melting point1133 K (859.85 °C) · estimated
Boiling pointNot available
Discovery (PubChem)1952

Values are reference data, not a specification for every temperature, pressure, or allotrope. Unknown properties are left unavailable. Superheavy-element predictions remain uncertain.

Sources & further reading

PubChem: Einsteinium · Einsteinium on Wikipedia

The introduction and original editorial story are adapted from Wikipedia contributors via Periodic-Table-JSON. Element descriptions and their adaptations are available under CC BY-SA 3.0. Numerical attributes use the PubChem periodic-table dataset, retrieved September 22, 2026.