Atomic Mass of Einsteinium (Es)
Einsteinium has no stable isotope, so no standard atomic weight exists; its most stable isotope has mass number ≈ 252, and the bracketed value [252] marks that estimate.
Looking for Einsteinium's atomic mass without wading through a whole chapter? Here is the direct answer first, then the tables and derivations examiners want to see. Every number on this page comes from the same engine that powers the interactive table. Need the whole picture instead — history, characteristics, uses, exam questions? The full Einsteinium page is one click away.
Einsteinium by the numbers
Z = 99 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average ([252] u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 99 |
| Relative atomic mass | [252] (no stable isotope) |
| Protons | 99 |
| Neutrons (commonest isotope) | 153 |
| Electrons | 99 |
Atomic mass vs mass number
Mass number (A) counts particles — protons + neutrons in one specific atom, always a whole number. Relative atomic mass is the experimentally measured average over all natural isotopes, so it comes out fractional ([252] is shown in brackets because it is only an estimate). Exams love this one-mark distinction.
Why is the mass in brackets?
Every isotope of Einsteinium is radioactive and vanishes too quickly to build up a natural mixture — so the IUPAC convention prints the mass number of the longest-lived isotope in square brackets instead of a standard atomic weight.
Students also ask
Is the atomic mass of Einsteinium a whole number?
No — Einsteinium has no stable isotopes, so chemists use the bracketed estimate [252] rather than a measured average.
How many neutrons does Einsteinium have?
Neutrons = mass number − atomic number. For Einsteinium's commonest isotope that is about 252 − 99 = 153, though the exact count differs between isotopes.