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99 californiumeinsteiniumfermium


Periodic Table - Extended Periodic Table
Name, Symbol, Number einsteinium, Es, 99
Chemical series actinides
Group, Period, Block n/a, 7, f
Appearance unknown, probably silvery
white or metallic gray
Atomic mass (252) g/mol
Electron configuration [Rn] 5f11 7s2
Electrons per shell 2, 8, 18, 32, 29, 8, 2
Physical properties
Phase solid
Density (near r.t.) 8.84 g·cm−3
Melting point 1133  K
(860 ° C, 1580 ° F)
Atomic properties
Electronegativity 1.3 (Pauling scale)
Ionization energies 1st: 619 kJ/mol
Magnetic ordering no data
CAS registry number 7429-92-7
Selected isotopes
Main article: Isotopes of einsteinium
iso NA half-life DM DE ( MeV) DP
252Es syn 471.7 d α 6.760 248Bk
ε 1.260 252Cf
β- 0.480 252Fm
253Es syn 20.47 d SF - -
α 6.739 249Bk
254Es syn 275.7 d ε 0.654 254Cf
β- 1.090 254Fm
α 6.628 250Bk
255Es syn 39.8 d β- 0.288 255Fm
α 6.436 251Bk
SF - -

Einsteinium ( IPA: /ˌʌɪnˈstʌɪniəm/) is a synthetic element in the periodic table that has the symbol Es and atomic number 99. A metallic highly radioactive transuranic element (7th in the series) in the actinides, einsteinium is produced by bombarding plutonium with neutrons and was discovered in the debris of the first hydrogen bomb test. It was named after Albert Einstein and has no known uses. Tracer studies using the isotope 253Es show that einsteinium has chemical properties typical of a heavy trivalent, actinide element.


Einsteinium was first identified in December 1952 by Albert Ghiorso at the University of California, Berkeley and another team headed by G.R. Choppin at Los Alamos National Laboratory. Both were examining debris from the first hydrogen bomb test of November 1952 (see Operation Ivy). They discovered the isotope 253Es ( half-life 20.5 days) that was made by the nuclear fusion of 15 neutrons with 238U (which then went through seven beta decays). These findings were kept secret until 1955 due to Cold War tensions, however.

In 1961, enough einsteinium was synthesized to prepare a macroscopic amount of 253Es. This sample weighed about 0.01 mg and was measured using a special balance. The material produced was used to produce mendelevium. Further einsteinium has been produced at the Oak Ridge National Laboratory's High Flux Isotope Reactor in Tennessee by bombarding 239Pu with neutrons. Around 3 mg was created over a four year program of irradiation and then chemical separation from a starting 1 kg of plutonium isotope.


19 radioisotopes of einsteinium have been characterized, with the most stable being 252Es with a half-life of 471.7 days, 254Es with a half-life of 275.7 days, 255Es with a half-life of 39.8 days, and 253Es with a half-life of 20.47 days. All of the remaining radioactive isotopes have half-lifes that are less than 40 hours, and the majority of these have half lifes that are less than 30 minutes. This element also has 3 meta states, with the most stable being 254mEs (t½ 39.3 hours). The isotopes of einsteinium range in atomic mass from 240.069 u (240Es) to 258.100 u (258Es).

Known Compounds

  • EsBr2 einsteinium(II) bromide
  • EsBr3 einsteinium(III) bromide
  • EsCl2 einsteinium(II) chloride
  • EsCl3 einsteinium(III) chloride
  • EsF3 einsteinium(III) fluoride
  • EsI2 einsteinium(II) iodide
  • EsI3 einsteinium(III) iodide
  • Es2O3 einsteinium(III) oxide
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