Atomic Mass of Neptunium (Np)
Neptunium has no stable isotope, so no standard atomic weight exists; its most stable isotope has mass number ≈ 237, and the bracketed value [237] marks that estimate.
Looking for Neptunium'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 Neptunium page is one click away.
Neptunium by the numbers
Z = 93 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average ([237] u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 93 |
| Relative atomic mass | [237] (no stable isotope) |
| Protons | 93 |
| Neutrons (commonest isotope) | 144 |
| Electrons | 93 |
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 ([237] 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 Neptunium 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 Neptunium a whole number?
No — Neptunium has no stable isotopes, so chemists use the bracketed estimate [237] rather than a measured average.
How many neutrons does Neptunium have?
Neutrons = mass number − atomic number. For Neptunium's commonest isotope that is about 237 − 93 = 144, though the exact count differs between isotopes.