---
title: "Einsteinium (Es) — element 99"
description: "Einsteinium is element 99, symbol Es, most stable isotope 252. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/einsteinium/
site: "At One Place"
---

# Einsteinium (Es)

**Einsteinium — element 99: Es**

[252] · Actinide · period 7

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

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 97 | Berkelium | Bk | [247] |
| 98 | Californium | Cf | [251] |
| 99 | Einsteinium ← | Es | [252] |
| 100 | Fermium | Fm | [257] |
| 101 | Mendelevium | Md | [258] |

## Common questions

### What is the atomic number of einsteinium?

99. That is the number of protons in its nucleus, which is what defines the element.

### What is the symbol for einsteinium?

Es.

### What is the electron configuration of einsteinium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f¹¹, or [Rn] 7s² 5f¹¹ in noble-gas shorthand.

### What is the melting point of einsteinium?

1,133 K — 859.9 °C — 1579.7 °F

## Related pages

- [Berkelium (Bk)](https://atoneplace.net/element/berkelium/)
- [Californium (Cf)](https://atoneplace.net/element/californium/)
- [Fermium (Fm)](https://atoneplace.net/element/fermium/)
- [Mendelevium (Md)](https://atoneplace.net/element/mendelevium/)

## Sources

- Official vendor product documentation — At One Place (/about/)