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Einsteinium is a chemical element with the atomic number 99. That means an Einsteinium atom consists of 99 protons, 100 or more neutrons and 99 electrons. It is a synthetic element, so all isotopes are radioactive.

Einsteinium belongs to the actinoids, and it is the 7th of them. The most stable isotope of Einsteinium consists of 99 protons and electrons and 153 neutrons. There are 19 different (radioactive) isotopes. Einsteinium is named after Albert Einstein. It was found for the first time in 1952 by Albert Ghiorso at the University of California, Berkeley.

Einsteinium istopes have a half-life between 40 and 471 days. No uses are known.