ActinideElement 99 · f-block

Einsteinium (Es)

Einsteinium (Es) is an actinide with atomic number 99; its most stable known isotope has mass number 252. At room temperature it is a solid that melts at 860 °C (estimated).

99EsEinsteinium[252]

Atomic number

protons

Atomic weight

[252]

most stable isotope

Melting point

860 °C

estimated

Boiling point

—

Density

8.84 g/cm³

predicted

Electronegativity

1.3

Pauling scale

State at 25 °C

Solid

Discovered

1952

United States

Einsteinium at a glance

Identity

Symbol	Es
Atomic number (Z)	99
Atomic weight	[252] mass number of the most stable isotope
Category	Actinide
Group	— (f-block: actinide series)
Period	7
Block	f-block
CAS number	7429-92-7

Atomic properties

Electron configuration	[Rn] 5f¹¹ 7s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹¹ 6s² 6p⁶ 7s²
Electrons per shell	2, 8, 18, 32, 29, 8, 2
Oxidation states	+3 Less common: +2, +4
Electronegativity (Pauling)	1.3
First ionisation energy	6.368 eV (614.5 kJ/mol) 2nd: 12.2 eV · 3rd: 22.7 eV
Covalent radius	165 pm
Van der Waals radius	245 pm

Physical properties

State at 25 °C	Solid
Melting point	860 °C · 1,580 °F · 1,133.15 K estimated
Density	8.84 g/cm³ (predicted)

Isotopes & abundance

Natural occurrence	Synthetic
Radioactive	Yes
Most stable isotope	²⁵²Es — half-life 471.7 days
Known isotopes	20

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Einsteinium electron configuration

Einsteinium has 99 electrons. Written in short form, its ground-state configuration is [Rn] 5f¹¹ 7s² — the electrons of the noble gas radon, plus 5f¹¹ 7s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹¹ 6s² 6p⁶ 7s².

5f↑↓↑↓↑↓↑↓↑↑↑

7s↑↓

Discovery and name

Discovered

1952 — Albert Ghiorso and co-workers (debris of the 'Ivy Mike' thermonuclear test) · United States.

Name

Albert Einstein.

Known for

Discovered in the debris of the first hydrogen-bomb test in 1952. Only microgram quantities have ever been made.

Isotopes of einsteinium

Einsteinium has no stable isotopes and does not occur naturally in significant amounts. Its longest-lived known isotope is:

Isotope	Protons	Neutrons	Half-life
²⁵²Es	99	153	471.7 days

20 isotopes of einsteinium are known in our data set (half-lives as published in nuclear data tables).

Einsteinium and its neighbours in the actinide series

Neighbouring elements in the same f-block series:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Curium	96	[247]	1,345 °C	—	13.51 g/cm³	1.3
Berkelium	97	[247]	986 °C	—	14.78 g/cm³	1.3
Californium	98	[251]	900 °C	—	15.1 g/cm³	1.3
Einsteinium	99	[252]	860 °C (est.)	—	8.84 g/cm³ (pred.)	1.3
Fermium	100	[257]	1,527 °C (est.)	—	9.7 g/cm³ (pred.)	1.3
Mendelevium	101	[258]	827 °C (est.)	—	10.3 g/cm³ (pred.)	1.3
Nobelium	102	[259]	827 °C (est.)	—	9.9 g/cm³ (pred.)	1.3

Frequently asked questions

6 answers

What is the atomic mass of einsteinium?

Einsteinium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [252], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does einsteinium have?

A neutral einsteinium atom has 99 protons and 99 electrons. Its most stable isotope, einsteinium-252, has 153 neutrons.

What is the electron configuration of einsteinium?

[Rn] 5f¹¹ 7s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹¹ 6s² 6p⁶ 7s²).

What are the melting and boiling points of einsteinium?

Einsteinium melts at 860 °C (1,580 °F) (estimated).

Is einsteinium a metal?

Einsteinium is a metal. It is in the f-block, period 7, and is a solid at room temperature.

Who discovered einsteinium?

1952 — Albert Ghiorso and co-workers (debris of the 'Ivy Mike' thermonuclear test) (United States).