Neptunium (₉₃Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be synthesized and identified was ²³⁹Np in 1940, produced by bombarding ²³⁸
U
with neutrons to produce ²³⁹
U
, which then underwent beta decay to ²³⁹
Np
.

Trace quantities are found in nature from neutron capture reactions by uranium atoms, a fact not discovered until 1951.

Twenty-five neptunium radioisotopes have been characterized, with the most stable being ²³⁷
Np
with a half-life of 2.14 million years, ²³⁶
Np
with a half-life of 154,000 years, and ²³⁵
Np
with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has five meta states, with the most stable being ^236m
Np
(t_1/2 22.5 hours).

The isotopes of neptunium range from ²¹⁹
Np
to ²⁴⁴
Np
, though the intermediate isotope ²²¹
Np
has not yet been observed. The primary decay mode before the most stable isotope, ²³⁷
Np
, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before ²³⁷
Np
are isotopes of uranium and protactinium, and the primary products after are isotopes of plutonium. Uranium-237 and neptunium-239 are regarded as the leading hazardous radioisotopes in the first hour-to-week period following nuclear fallout from a nuclear detonation, with ²³⁹Np dominating "the spectrum for several days."