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.

Recognizing Einsteinium in Elementle

If Einsteinium is the mystery element, guesses below atomic number 99 point up and guesses above it point down. Its symbol is Es: a green letter matches the same position in that symbol, while a yellow letter belongs in another position.

The family clue turns green for actinide guesses. Compare Californium (Cf), number 98 and Fermium (Fm), number 100: they share the family clue, but their atomic numbers and symbols help you tell them apart.

Learn how to read the clues or play the daily element guessing game.

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.