One of the most frustrating mysteries of modern physics has finally been solved. Scientists have been puzzled for decades why plutonium, which is a metal, does not have the same magnetic properties that other metals do.

Researchers at Los Alamos National Laboratory discovered that unlike other metal elements, an atom of plutonium has a variable number of electrons in its outer shell which makes it impossible for magnetism to be displayed.

The research was published in the July 10th issue of Science Advances.

Plutonium atoms don’t line up to crate a magnetic field

Magnetic attraction is created by the attractive force of unpaired electrons. Each electron is like a micro-magnet with a north and a south pole that is attracted to line up with another electron (creating the electron shell around an atom). For example, when iron is in a magnetic field the unpaired electrons all line up the same direction, producing an aggregate magnetic field around the iron and attracting other magnets.

Despite being a metal like iron, plutonium does not produce an aggregate magnetic field when placed in a magnetic field because the electron shell of plutonium is constantly shifting between four, five and six electrons in its shell. This means the electrons in plutonium never line up to create a magnetic field, even when in the presence of another magnet.

Plutonium creates new category on the periodic table

According to lead researcher Marc Janoschek, plutonium’s properties place the element between transition metals and rare earths on the periodic table. “Look at thorium to uranium and neptunium — they behave like transition metals, they get more metallic” he said. As you move to heavier elements (to the right on the periodic table), that changes. “When you get to americium and beyond that they look like rare earths.”

Of note, rare earths can be used to create magnets, while transition metals often cannot.

New research is big step forward in materials science

The new technique developed for the experiment along with the discovery of plutonium’s unique electron shell will be useful to scientists in projecting the properties of new materials. To date, aside from some limited computer modeling, the only way to find out the behaviors of new materials was to undertake experiments such as heating them up or applying electricity or magnetic fields. This new research provides a way to learn a lot about a material without actually conducting experiments.

“A predictive theory of materials is a big deal because we eventually will be able to simulate and predict properties of materials on a computer,” noted Gabriel Kotliar, a professor of physics at Rutgers and a key researcher on the project. “For radioactive materials like plutonium, that’s a lot cheaper than doing an actual experiment.”

This new understanding of plutonium also explains why the radioactive element expands and contracts much more than other metals do when heated or when an electric current is applied. This new research also explains why plutonium has to be so precisely shaped in order for it to be used in a nuclear bomb. Nuclear engineers learned to account for the shape changing of plutonium decades ago, but they finally know why the element exhibits such strong shape-changing properties.