An Overview about the Chemical Element Neptunium


Symbol: Np

Atomic Number: 93

Atomic Mass: 237 amu (atomic mass units)

Melting Point: 640.0 C (913.15 K, 1184.0 F)

Boiling Point: 3902.0 C (4175.15 K, 7055.6 F)

Number of Protons: 93

Number of Electrons: 93

Number of Neutrons: 144

Classification: Rare Earth

Crystal Structure: Orthorhombic Man made

Density @ 20 C (293 K, 68 F): 20.45 grams per cubic centimeter

Color: Silver

Neptunium was the first of the man made transuranium elements to be produced. It was made in 1940 by Edwin M. McMillian and Philip H. Abelson at the University of California, Berkeley, California, USA. They had produced slow moving neutrons with a cyclotron which they used to bombard a uranium target. The isotope produced in this experiment was neptunium-239 which has a half-life of 2.4 days. The element is named after the planet Neptune.

Neptunium is a member of the actinide or actinoid series of rare earth elements. It is radioactive so must be handled using the appropriate safety measures.

The element has at least three temperature dependent allotropes or crystal forms. At room temperature alpha-neptunium exists and has an orthorhombic crystal form. Above 288 C (516 K, 550.4 F) it becomes beta-neptunium with a tetragonal crystal form and a density @ 313 C (586 K, 595.4 F) of 19.36 grams per cubic centimeter. While above temperatures of 577 C (850 K, 1070.6F) it becomes cubic gamma-neptunium which has a density @ 600 C (873 K, 1112 F) of 18 grams per cubic centimeter.

Neptunium also has four different ionic oxidation states. These states can be distinguished from each other by the colors of their solutions. The Np3+ state is pale purple, Np4+ is yellow-green, Np(O2)+ is green-blue and Np(O2)2+ is pale pink.

There are no stable isotopes of neptunium. A number of unstable isotopes have been produced with mass numbers ranging from 225 to 244. The most stable isotope is neptunium-237 which has a half-life of 2,144,000 years. Neptunium-237 decays by alpha decay to form protactinium-233.

The supply of neptunium comes from nuclear reactors where neptunium-237 is a by-product from the production of plutonium. Pure neptunium is obtained by the reduction of neptunium tri-fluoride at 1200 C (1473 K, 2192 F) using either lithium or barium. It is used in the production of neutron detectors.

While neptunium is considered a man-made element there is evidence of very small quantities of the element being present in uranium ores. It is produced naturally in these ores by neutrons from uranium decay impacting on other uranium atoms.