Atomic number 99

Einsteinium

Es
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Shells Orbitals 5f 7s

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99 electrons in 7 shells: 2, 8, 18, 32, 29, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Actinide
Period: 7
Block: f
Atomic mass: 252.0830 Da (longest-lived isotope)
Electron configuration: [Rn] 5f¹¹ 7s²
Phase at 25 °C: solid
Melting point: 860 °C
Boiling point: 996 °C
Electronegativity (Pauling): 1.3
Discovered: December 1952, Ghiorso and co-workers at Berkeley

Neighbours in the table

Cf Fm

Einsteinium carries the name of Albert Einstein. It first turned up in December 1952, in the debris of the earliest large thermonuclear explosion, fired in the Pacific a month before. The identification was the work of Ghiorso and co-workers at Berkeley, in Los Alamos’s account. The discoverers came from three American laboratories, Argonne, Los Alamos and Berkeley, in the account of IUPAC, the International Union of Pure and Applied Chemistry. Los Alamos counts it the seventh transuranic actinide discovered.

Its configuration is [Rn] 5f¹¹ 7s². Among the actinides, einsteinium is the first metal to be divalent, in Los Alamos’s words, bonding with two electrons rather than three. Tracer studies with einsteinium-253, which follow trace amounts of an element by its radioactivity, still showed the chemistry of a heavy trivalent actinide. An oxidation state is the charge an atom would carry if every bond were ionic. Einsteinium’s are 2 and 3 for Los Alamos, with 4 proposed from vapour transport studies but never firmly established; for Jefferson Lab the state is +3.

Ionisation energy is the work needed to pull one electron away from a neutral atom against the attraction of its nucleus. Einsteinium’s is 6.36840 eV in NIST’s figure and 6.42 eV in Jefferson Lab’s. PubChem gives the mass of its longest-lived isotope, einsteinium-252, as 252.0830 Da.

Where it is found

Einsteinium is absent from the Earth’s crust in nature, IUPAC states, and none of its isotopes is stable. The discovery isotope, einsteinium-253, lasts 20.47 days before half of a sample has decayed, for the International Atomic Energy Agency (IAEA). IAEA’s evaluation gives alpha emission for 100 per cent of its decays and spontaneous fission, in which a nucleus breaks apart by itself, for 0.0000087 per cent.

In Jefferson Lab’s description, each isotope in a long chain of nuclear reactions is struck by neutrons, and the isotope that forms then undergoes beta decay. Oak Ridge’s High Flux Isotope Reactor (HFIR) has made about 3 µg of the element, according to Los Alamos. Plutonium is irradiated with neutrons for several years, Los Alamos records. Targets made from it then spend a year at the Savannah River Plant and four months in the HFIR. Chemists then separate the einsteinium from the californium formed alongside it. Special HFIR campaigns can hold about 2 milligrams, Los Alamos adds.

Why it matters

By 1961 enough einsteinium existed to separate a macroscopic sample of einsteinium-253, Los Alamos records. It weighed about 0.01 µg on a special magnetic-type balance. Einsteinium-253 made this way was then bombarded with neutrons to create mendelevium, element 101. Einsteinium has no commercial uses, yet it is among the heaviest elements on which bulk studies can be performed, Los Alamos says. Those studies probe the part 5f electrons play in the trends across the actinide series.

Einsteinium-253 and einsteinium-254 are the isotopes of choice for physical and chemical work, being available and reasonably long-lived, according to Los Alamos. Experiments use only very small samples, which reduces worker exposure and limits the intense self-irradiation. That self-irradiation defeats x-ray crystallography, the usual way to map atoms in a solid. Gamma rays and x-rays from its daughter products, the nuclei its decay produces, over-expose the film or detector. The same damage can be turned to use in studies of accelerated ageing and radiation damage, and, according to Los Alamos, in targeted radiation medical treatments.

Einsteinium-253’s fairly short half-life lets chemists watch its daughter berkelium-249 build up, then the granddaughter californium-249. Los Alamos gives their half-lives as 330 days and 351 years. Evidence suggests divalent einsteinium might decay into divalent berkelium, and then into a divalent californium not yet known, Los Alamos says.

Properties

Properties of einsteinium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	99
Atomic mass	252.0830 Da	Da	PubChem, longest-lived isotope
Melting point	860 °C	°C	Los Alamos: 860 °C
Boiling point	996 °C	°C	Los Alamos: 996 °C
Electronegativity (Pauling)	1.3
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f¹¹
Phase at 25 °C	solid

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Einsteinium is a radiation hazard, handled only in research laboratories. In Haz-Map’s summary, carried by the PubChem Laboratory Chemical Safety Summary, einsteinium-254 is the most important isotope, rated very high for internal toxicity, the harm done inside the body. Inhaled einsteinium clears from the lungs with a half-time of weeks, Haz-Map adds. The radiation and heat its isotopes give off make them difficult to use in experiments, IUPAC notes.

Questions

How was einsteinium discovered?

A team led by Albert Ghiorso found it in 1952 while studying radioactive debris from the first hydrogen bomb, according to Jefferson Lab. The isotope they identified, einsteinium-253, had formed when uranium-238 absorbed 15 neutrons and then passed through seven beta decays. That isotope was the first einsteinium isotope identified, IUPAC notes, and its half-life is 20.47 days in the International Atomic Energy Agency's evaluation.

What is the longest-lived isotope of einsteinium?

Einsteinium-252, whose half-life is 471.7 days in the evaluation of the International Atomic Energy Agency (IAEA) and for Jefferson Lab. IAEA assigns 78 per cent of its decays to alpha emission, giving berkelium-248, and 22 per cent to electron capture, giving californium-252. PubChem gives the mass of this isotope as 252.0830 Da.

What is einsteinium used for?

Only research. The reason, for Jefferson Lab, is the small amounts ever produced. Its research uses, in IUPAC's words, are making still heavier transuranic elements and studying actinide science. Einsteinium-253 was used to make mendelevium, element 101, by neutron bombardment, Los Alamos records. Einsteinium has no commercial uses at all, according to the same laboratory.

Is einsteinium dangerous?

Yes, as a source of ionising radiation. In the Haz-Map summary carried by PubChem, einsteinium-254 is rated very high for internal toxicity, the harm done once it is inside the body. No everyday form exists: according to IUPAC, it is not found naturally in the Earth's crust. Laboratories keep samples very small to limit worker exposure, Los Alamos says.