Einsteinium (Es)
Element 99 · Actinide · atomic mass [252] u · f-block, period 7
Electron shells: 2, 8, 18, 32, 29, 8, 2
Einsteinium was found in the fallout of the first hydrogen bomb test in 1952. It is so radioactive that it damages its own crystal structure as you study it.
Its discovery was classified for three years because it revealed details of hydrogen bomb design.
- Atomic number
- 99
- Atomic mass
- [252] u
- Electron configuration
- [Rn] 7s² 5f¹¹
- Group · period · block
- f-block · 7 · f
- Oxidation states
- +3
- Electronegativity
- 1.3 (Pauling)
- Melting point
- 860 °C
- Boiling point
- —
- Density
- Not measured
- State at 20 °C
- Solid
- Appearance
- Silvery metal
- Discovered
- Albert Ghiorso and colleagues, 1952
99 protons, and 99 electrons when neutral
Mass number of the longest-lived isotope — no standard atomic weight exists
Shown in the strip below the table
What Einsteinium is used for
- Scientific research
- Producing heavier elements
Atomic structure of Einsteinium
A neutral einsteinium atom has 99 protons and 99 electrons. Its most common isotope carries about 153 neutrons, which is where the mass number 252 comes from. Change the neutron count and you have a different isotope of einsteinium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 32, 29, 8, 2. In orbital notation that is [Rn] 7s² 5f¹¹. As a f-block element its valence count is variable, which is why it can form compounds in several oxidation states.
3D electron orbitals
See where Einsteinium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Einsteinium sits in the periodic table
The second f-block row, elements 89-103. All are radioactive, and everything past uranium is essentially man-made.
- One proton lighter: Californium.
- One proton heavier: Fermium.
Discovery and naming
Einsteinium was discovered by Albert Ghiorso and colleagues in 1952. Its name comes from Albert Einstein, which is also why its symbol (Es) does not match its English name.
Every isotope of einsteinium is radioactive, so it has no standard atomic weight. Periodic tables show the mass number of its longest-lived isotope in square brackets instead.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Einsteinium?
Einsteinium has no stable isotope, so it has no standard atomic weight. Periodic tables instead show 252 in square brackets — the mass number of its longest-lived isotope.
How many protons, neutrons and electrons does Einsteinium have?
Einsteinium has 99 protons, because the atomic number is the proton count and that is what makes an atom Einsteinium at all. A neutral Einsteinium atom has the same number of electrons, 99. The neutron count varies by isotope; the most common one has about 153, since 252 − 99 = 153.
What is the electron configuration of Einsteinium?
The electron configuration of Einsteinium is [Rn] 7s² 5f¹¹. Its electrons are arranged 2, 8, 18, 32, 29, 8, 2 across 7 shells.
Is Einsteinium a metal?
Yes. Einsteinium is classified as a actinide, on the metallic side of the table.
What group and period is Einsteinium in?
Einsteinium sits in the f-block strip below the main table, in period 7, among the actinides. Elements in that strip are not given a group number in the standard 18-column layout.
What is the charge of Einsteinium?
Einsteinium most commonly forms a 3+ ion. Like most transition metals it has several accessible oxidation states, so the charge depends on the compound.
What is the melting point of Einsteinium?
Einsteinium melts at 860 °C. That puts it in the solids at everyday temperatures.
What is Einsteinium used for?
Scientific research, Producing heavier elements. Einsteinium was found in the fallout of the first hydrogen bomb test in 1952.
Who discovered Einsteinium, and where does its name come from?
Einsteinium (Es) was discovered by Albert Ghiorso and colleagues in 1952. The name comes from Albert Einstein.
Is Einsteinium radioactive?
Yes. Every isotope of Einsteinium is radioactive — it has no stable form at all, which is why the periodic table shows its mass in square brackets rather than as a standard atomic weight.