Einsteinium (Es)

Einsteinium and fermium were found in the fallout of the Ivy Mike thermonuclear test in the Pacific in 1952 — uranium in the device had absorbed a burst of neutrons and climbed the periodic table in a fraction of a second — and the discovery stayed classified for three years. The longest-lived isotope lasts 471 days, and the world has never held more than a few milligrams at once. Its own radiation destroys the crystal lattice of any compound it forms, which is why measuring even basic chemistry took until 2021 to do properly.

Properties

Atomic number

99

Symbol

Es

Mass number of most stable isotope

[252]

Category

Actinide

Period

7

Block

f-block

Electron configuration

[Rn] 7s² 5f¹¹

Electrons per shell

2, 8, 18, 32, 29, 8, 2

Melting point

1,133 K — 859.9 °C — 1579.7 °F

Electronegativity

1.3 (Pauling)

Oxidation states

+3

Discovered

1952 by Albert Ghiorso and colleagues at Berkeley and Los Alamos

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f¹¹

Noble gas shorthand

[Rn] 7s² 5f¹¹

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 29 · P: 8 · Q: 2

What it is used for

• Making mendelevium, and fundamental studies of heavy-element chemistry

Hazards

• Intensely radioactive; a milligram-scale sample generates visible light and significant heat

No stable isotope; the mass given is the mass number of einsteinium-252, the longest-lived at 471 days. Boiling point and density are omitted: no measured values exist. The electronegativity of 1.3 is the nominal figure CRC assigns to all the later actinides rather than an independent measurement.

Several physical properties are not listed for einsteinium because no bulk sample has ever existed. Only a handful of atoms have been produced, often with half-lives measured in seconds, so published melting points and densities for elements in this range are calculations rather than measurements. They are omitted rather than presented as observed values.

Neighbours in the table