The Element of the Month - Neptunium

Author: Simon Moorhouse
Date: Apr 27, 2016


Welcome to the 11th instalment of the Element of the Month – a regular feature on the Radleys blog.

As usual, we chose our element of the month by using a random number generator to produce a figure between 1 and 118 – the number of elements in the periodic table (that we know of).

This month we’re looking at atomic number 93, neptunium (Np).

Neptunium – The Key Facts

Neptunium is a radioactive metal in the actinide series.

Its name is derived from the planet Neptune. Neptune is the next planet out from the Sun after Uranus, just like neptunium is the next element in the periodic table after uranium.

This silvery solid is thought to have the widest difference of any element between its melting point (640°C) and boiling point (estimated at 4174°C).

Neptunium has three different possible allotropes (structures), depending on its temperature. It has five oxidation states, +3 to +7, each with a different colour in solution, ranging from violet to yellow green.

It has no stable isotopes. Instead, it has 20 radioisotopes with a range of half-lives and mass numbers 225 to 244. Its longest living isotope is neptunium-237, with a half-life of 2.14 million years.

The main use of neptunium-237 is in devices for detecting high-energy neutrons.

Neptunium is dangerous. As well as radioactive, it's also pyrophoric, capable of spontaneously catching fire at room temperature.

A Few Fascinating Facts about Neptunium

Neptunium was discovered in 1940 by Edwin McMillan and Philip H. Ableson at the Berkeley Radiation Laboratory in California. They synthesised neptunium-239 (half-life of two and half days) through bombarding uranium with neutrons from a cyclotron particle accelerator.

Neptunium was the first transuranium element, with an atomic mass heavier than uranium, once thought to be the heaviest element. 

McMillan and Ableson succeeded where others had failed – Enrico Fermi had previously reported generating element 93, and was even awarded the 1938 Nobel Prize in Physics partly for “for his demonstrations of the existence of new radioactive elements produced by neutron irradiation”, before embarrassingly evidence emerged that his data had been misinterpreted.

It was once believed that neptunium could only be produced artificially, but it's since been discovered in trace amounts in uranium ores, where its presence can be attributed to decay. Most of today’s neptunium is a by-product of neutron irradiation of uranium in nuclear power reactors, and much of the neptunium present in the environment was generated by nuclear explosions.

Like its more famous neighbours in the periodic table, uranium and plutonium, neptunium can form the basis of an atomic bomb. The main reason it wasn't used in nuclear weapons is simply that uranium and plutonium research was already established before the relatively stable neptunium-237 isotope was discovered.

A key potential use of neptunium is to produce nuclear batteries for spacecraft, a topic we covered in this month’s Week in Chemistry blog post. It’s actually plutonium-238 that’s in these generators, its radioactive decay producing heat – a compact, reliable energy source that’s vital for many NASA missions. 

Plutonium-238 was originally obtained in the US from nuclear reactors, but after they were shut down, and supplies dwindled, NASA started looking for a new source. Scientists recently announced that they have succeeded in producing neptunium-238 by bombarding pellets of neptunium-237 oxide and aluminium with neutrons, which then transforms into the prized plutonium-238 by beta decay.

In a more everyday setting, you may find neptunium in your smoke detector – although it wasn't deliberately placed there. Ionisation-type smoke detectors use tiny amounts of the radioisotope americium-241, but thanks to alpha emissions, over time this decays into neptunium-237. In around 50 years, the radioactive component would be 10% neptunium.

So that’s neptunium – an elusive radioactive element that contributed to an arguably mistaken Nobel Prize, that could play an important role in spaceflight, and that may well be hiding in your home.

Join us next month, when we’ll be exposing californium to considerable scrutiny.

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