"Neptunium (
/nɛpˈtjuːniəm/ nep-tew-nee-əm) is a chemical element with the
symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element, and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutoniumproduction, and it can be used as a component in neutron detection equipment. Neptunium is also found in trace amounts in uranium ores due to transmutation reactions."
"The periodic table of Dmitri Mendeleev published in the 1870s showed a " — " in place after uranium similar to several other places for at that point undiscovered elements. Also, a 1913 publication of the known radioactive isotopes by Kasimir Fajans shows the empty place after uranium."
"At least three times, discoveries of the element 93 were falsely reported, as bohemium and ausonium in 1934, and then sequanium in 1939. The name neptunium had previously been considered for germanium."
"The search for element 93 in minerals was encumbered by the fact that the predictions on the chemical properties of element 93 were based on a periodic table which lacked the actinide series, and therefore placed thorium below hafnium, protactinium below tantalum, and uranium below tungsten. This periodic table suggested that element 93, at that point often named eka-rhenium, should be similar to manganese or rhenium. With this misconception it was impossible to isolate element 93 from minerals, although neptunium was later found in uranium ore, in 1952."