Einsteinium at a glance
- Atomic number
- 99
- Symbol
- Es
- Atomic mass
- 252 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 5f 11 7s 2
- State at 25 °C
- Solid
- Melting point
- 859.9 °C
- Boiling point
- 995.9 °C
- Density
- 8.84 g/cm³
- Electronegativity
- 1.3
- 1st ionization energy
- 619 kJ/mol
- Oxidation states
- +3
- Discovered
- 1952
Atomic structure
A neutral einsteinium atom has 99 protons and 99 electrons, and its most common isotope carries about 153 neutrons. One atom weighs about 4.18 × 10⁻²⁵ kg, so a mole of einsteinium (6.022 × 10²³ atoms) weighs 252 g. That nucleus holds 1.55 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 7 shells (K 2, L 8, M 18, N 32, O 29, P 8, Q 2), which gives the configuration [Rn] 5f 11 7s 2. As a f-block element, both its outer s electrons and its inner f electrons can take part in bonding, which is why it shows more than one oxidation state. All of its isotopes are radioactive, so any einsteinium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) einsteinium is a solid. It melts at 859.9 °C and boils at 995.9 °C, so it stays liquid across a 136-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 8.84 g/cm³, 8.84 times as dense as water. In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 1.3 on the Pauling scale. Pulling off its outermost electron takes 619 kJ/mol. A bond between einsteinium and chlorine would be mostly ionic, with an electronegativity difference of 1.9. Its common oxidation states are +3, so in compounds you will usually meet it as Es³⁺. It shares much of its chemistry with the other actinides, especially its neighbours Berkelium, Californium, Fermium and Mendelevium. Handle with care: as a pure element it is radioactive.
Where it comes from
It does not occur in nature in useful amounts; it is made in nuclear reactors or particle accelerators. It was first made in the fallout of the 1952 'Ivy Mike' thermonuclear test by uranium soaking up many neutrons in a split second. Its longest-lived known isotope is Es-252, about 470 days. It was first identified in 1952, making it the 99th element to be discovered (counting the 11 known since antiquity together).