Plutonium is puzzling scientists. It’s one metal that just won’t jam well with a magnet. And now we know the reason behind the “missing magnetism”: electrons that surround every atom of plutonium. The study published in today’s (July 10) issue of Science Advances reveals that because the number of electrons in plutonium’s outer shell keeps changing, the unpaired electrons will never line up in a magnetic field making it impossible for plutonium to become magnetic.
To crack the secret behind the mystery, Janoschek’s team fired a beam of neutrons at a plutonium sample – to demonstrate how plutonium’s electrons looked like in this ground state. The neutrons and electrons both have magnetic fields, and those fields have magnetic moments. A magnetic moment refers to the amount and direction of the force needed to align an object in a magnetic field. As the neutrons’ and electrons’ moments interacted, Janoschek’s team observed a kind of signature of the electrons’ ground states, which revealed the number of electrons in the outer shell. That’s when they found plutonium could have four, five or six electrons in the outer shell in the ground state. Scientists who were trying to explain the element’s odd properties previously had assumed the number was fixed.
But that’s not what the new study showed. “It fluctuates between the three different configurations,” Janoschek said. “It is in all three at the same time.”
What are Electrons?
Electrons spin around atoms in shells – also called orbitals. Each orbital has a certain maximum number of electrons it can hold. In ordinary metals the number of electrons in the outermost orbital is fixed — copper, for example, has one electron, and iron has two in that outer shell. With the absence of any external energy, these electrons are usually in their ‘ground state’ – a state of lowest energy.
The theoretical basis for this mind-boggling concept was laid in 2007, when physicists at Rutgers University developed a new mathematical tool that assumed plutonium’s electrons could fluctuate in this way. The Los Alamos experiment is the first test of the theory, and it has proven correct.
Interestingly, magnets get their ability to adhere from ‘unpaired electrons’. To explain further, each electron is like a tiny magnet with a north and a south pole. When electrons fill an atom’s shells, they each take their place singly, and the magnetic moments point in the same direction. As more electrons fill the shell, they pair up with the north and south poles each facing each other so that the magnetic fields cancel out. However, sometimes an electron can’t find a partner. For instance, when iron is put in a magnetic field the unpaired electrons all line up the same way, creating an aggregate magnetic field and attracting other magnets.
Janoschek said plutonium’s properties place the element between two sets of elements 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.” Rare earths like neodymium make very good magnets, while transition metals often don’t.
The experiment did more than just notch another weird property of plutonium. The mathematical technique in the experiment along with the discovery of plutonium’s weird electrons could help scientists predict how new materials might behave. Up until now the only way to pin down this behavior was to do experiments like heating them up or hitting them with electricity or magnetic fields. Now there’s a way to know beforehand.
“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,” said Gabriel Kotliar, a professor of physics at Rutgers and one of the scientists who first worked out the mathematics. “For radioactive materials like plutonium, that’s a lot cheaper than doing an actual experiment.”
The research has helped scientists understand another eccentric property of plutonium — the element expands and contracts a lot more than other metals when heat or electricity is applied. This characteristic is the key when it comes to constructing nuclear bombs.