Einsteinium (Es)
Element 99 · Actinide · Solid at 25 °C · Period 7 (f-block)
Einsteinium was discovered in the radioactive debris of the first hydrogen-bomb test in 1952 and named for Albert Einstein.
On the periodic table, einsteinium sits in the f-block in period 7. It is one of 15 elements classified as an actinide, and at room temperature (25 °C) it is a solid.
The actinides fill the 5f orbitals in the bottom row of the table, and every one of them is radioactive. Only thorium and uranium occur on Earth in significant quantities; the rest are made in reactors or particle accelerators. Their nuclear properties — fission above all — shaped the twentieth century.
Properties of einsteinium
| Atomic number (Z) | 99 |
|---|---|
| Symbol | Es |
| Atomic mass | (252) (mass number of the most stable isotope; this element has no standard atomic weight) |
| Category | Actinide |
| Group | — (f-block, associated with group 3) |
| Period | 7 |
| Block | f-block |
| Metal / nonmetal | Metal |
| State at 25 °C | Solid |
| Electron configuration | [Rn] 5f11 7s2 |
| Electronegativity (Pauling) | 1.3 |
| Melting point | 860 °C (1,133.15 K / 1,580 °F) |
| Boiling point | — |
| Density | 8.84 g/cm³ |
Position in the periodic table
Einsteinium belongs to the actinides, the detached f-block row shown beneath the main table. These elements all fit between groups 2 and 4 of period 7, and are pulled out into their own row only to keep the table a manageable width.
Physical properties
Its density is 8.84 g/cm³.
Data notes
The atomic mass is shown in parentheses because this element has no standard atomic weight; the number given is the mass number of its most stable known isotope.
See also: the interactive periodic table, how to read the periodic table, periodic trends.