SCIENCE / PHYSICS : New Element (No. 111) Stretches Limits of Atom
Every hamburger and Volkswagen, every newborn child and billion-year-old planet, is fashioned from the same elementary ingredients. The difference is only: how much, how many, in what combination?
Chemists are constantly trying to understand the architecture of atoms, seeking to discover, or even create, new elements. So far they have come up with 19 beyond the 92 elements that occur in nature.
In an impressive feat of modern-day alchemy, scientists have forged the latest new element--the heaviest yet--by mating nickel and bismuth atoms in an accelerator in Germany. The brief union that produced element 111 broke up after a mere two-thousandths of a second, but not before giving birth to several generations of daughter and granddaughter atoms.
The new element is especially intriguing to scientists because such a ponderous nucleus, by some accounts, should not have held together at all. The fact that it did gives hope that a new family of stable elements might be waiting to be discovered around element 114.
UC Berkeley’s Glen Seaborg, who won the Nobel Prize in 1951 for his part in creating heavy elements, said, “We’re very pleased. . . . This (discovery) means it will be possible to go and make even heavier elements.”
Other scientists are simply glad to admit a new member to the chemical family. “The most exciting thing for me is always to make a new element,” said UC Berkeley nuclear chemist Darleane Hoffman, who has been involved in this kind of nuclear matchmaking since post-World War II days. “The whole thing is so amazing.”
Every element, from elusive helium to glowing gold, is composed of three basic parts: protons, neutrons and electrons. But it is the positively charged proton that anchors the nucleus and determines how many negatively charged electrons can be held in its grasp, and also how many neutral neutrons might hang around to add heft.
The proton number determines an element’s identity. Hydrogen, with 1 proton, is element 1. Helium, with two protons, is element 2. And so forth.
Making new elements tells scientists something about the basic forces that hold atoms--and everything made of them--together. The same processes that create new elements fuel stars and nuclear power plants, and they determine the properties of all materials.
But creating heavier elements is extremely difficult because of the inherent instability of all elements beyond 92 (uranium).
Because all protons are positive, and like charges repel, more protons make for more repulsive forces. At very intimate distances, this repulsion is balanced by the strong attraction that all nuclear particles feel for each other (known as the “strong” force). But this love-hate relationship makes very large atoms very unstable. The heavier they are, the sooner they fall apart, spitting out their nuclear particles bit by bit until they settle down to smaller, more stable, forms. Such decaying elements are said to be radioactive.
The union made in Germany at the Society for Heavy Ion Research (GSI) last month was successful because the team was able to get a nickel atom to snuggle up to a bismuth atom with exactly the right energy, and because the group had a detector sensitive enough to catch the new element in the act of decaying. The team used the same technique in November to produce element 110. Neither new element yet has a name.
The merger also succeeded because of stabilizing “shell effects” predicted to exist at element 114, but whose effects are already beginning to kick in at 111.
Both the wispy electrons that buzz in a continual blur around the periphery of an atom, and the nuclear particles within its core, are arranged in concentric shells, like so many subatomic Russian dolls. Quality follows directly from quantity. If the atom has enough particles to fill its shells, it becomes stable--like a table with four legs. If it has too few or too many, it wobbles, changing into another form.
For example, element 10 is neon. With two full shells of electrons, it is so stable it does not interact with other elements, earning its title as a “noble gas.” Element 11 is sodium. The lone extra electron on its outermost border makes it eager to merge, and highly reactive.
Going beyond 100, said Hoffman, “depends on the stabilizing effect of the shells.” The Berkeley group created elements 101 through 106--the latter initially was named Seaborgium in honor of Seaborg, but now the name is in dispute. The German group created 107 through 111.
When the experimenters in Germany beamed a stream of nickel atoms at a bismuth target, most encounters came to naught. Either the atoms missed or embraced so energetically that they simply split apart before fusing into a new element. Only three managed to merge long enough to be measured.
In those isolated cases, the new 111 atom almost immediately spit out an alpha particle (or a helium nucleus), losing two protons and two neutrons and turning into element 109. Then 109 spit out another alpha particle and became 107. The chain of generations is what established the existence of the new element beyond a doubt.
“I think it’s nice that they call them daughters and granddaughters,” Hoffman said. “There were a lot of famous women nuclear physicists.”
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An Elementary Process
Here is how scientists formed element 111:
* 1. Argon atoms are shot at a rare isotope of nickel to knock off nickel atoms.
* 2. Some electrons are knocked off so the nickel atoms carry an electric charge, making it possible to accelerate them with an electric field.
* 3. The atoms are sent through a series of accelerators that zap them with periodic electrical pulses, pushing them faster and faster until they are traveling at 10% of the speed of light.
* 4. The atoms are steered toward their target--in this case, a thin foil sheet of bismuth metal. In order to merge with the bismuth in a new atom, the nickel atoms must strike the target with exactly the right energy.
* 5. A successfully merged atom then recoils and travels through a 12-meter-long separator that picks out only atoms traveling at velocity predicted for the new element. The newly merged atom will travel more slowly than the other recoiling nickel atoms because it is more massive.
* 6. The atom is steered into a box, roughly the size of a bar of soap, covered with silicon detectors.
* 7. The detector absorbs the alpha particles that the atom spits out, recording where they hit, and when. This tells researchers the identity of the new element.