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A Noble Gas Surprise: Helium Can Form Weird Compounds
A new idea explains recently discovered chemistry that seems to break the rules of high school
textbooks everywhere
By Clara Moskowitz on March 20, 2018
Helium, the most noble of the noble gases, long thought to be completely inert and thus too
standoffish to bond with other atoms, recently surprised chemists by forming chemical compounds
after all.
Last year scientists reported producing the compounds—crystals made of sodium and helium
atoms—but could not understand how they formed. Now a new team of researchers has offered an
explanation: Helium manages to combine with other atoms without making any chemical bonds—that
is, without sharing or exchanging any electrons. The element does this by shielding positively charged
atoms from each other, acting as a buffer between their repellent charges.
The compounds are stunning because scientists had thought helium extremely unlikely to
combine with other atoms. This is because a helium atom is unwilling to give up its two electrons, which
perfectly fill its only electron shell. Every atom has such shells, which hold specific numbers of electrons
and structure these negatively charged particles around the atomic nucleus. Atoms prefer their shells to
be totally full, and will bond with other atoms that can take or give an extra particle or two to fill out a
shell. Elements with shells that are already full and have no electrons to lend are called noble gases—
and helium, the smallest of these, is considered the most inert. “And then came this wonderful work last
year,” says California State University, Northridge, chemist Maosheng Miao, leader of the team that
offered the new explanation. Miao’s graduate student, Zhen Liu, was lead author of the paper, which
was published March 5 in Nature Communications. “They found that if you put sodium and helium
together and compress it to pressures like at the center of the Earth, sodium can actually react with
helium and form stable compounds.” At first some scientists thought helium might be sharing electrons
after all. But Miao’s team suggested an alternative explanation: Maybe helium is not giving or receiving
any electrons, but is somehow combining with sodium anyway.
High-enough pressure can crush a collection of sodium atoms to the point that the extra
electron on each atom gets squeezed out, turning all the atoms into positively charged ions. Each ion
then repels all its neighboring ions, because like charges push against each other. Miao and his
colleagues reasoned that if helium atoms could come and sit in between sodium ions, the distance
between the positive charges would increase—and the repulsive energy would lessen, stabilizing the
material. “I think this is the first time ever that there’s no chemical bond involved, and yet you can form
a stable compound,” Miao says.
Based on their hypothesis, Miao’s team performed exhaustive computer calculations using the
quantum mechanical laws governing each atom, and found that indeed such compounds should work.
The discovery may also have implications for the composition of elements thought to exist deep
inside our planet. Scientists had thought helium, lacking a way to bond with other elements, could not
possibly be locked away inside Earth’s rocks. “Now it becomes increasingly clear that this is a gross
oversimplification,” Oganov says. “Even helium, the most inert of all, is not actually as inert as we
thought. It can actually form stable compounds and be retained in the Earth’s mantle.”
In the future chemists would like to find more general rules to predict when such unusual
molecules could arise, because under high pressure many of the normal laws of chemistry do not apply.
Questions:
1. What did Maosheng Miao’s chemistry team discover?
2. Why is this discovery surprising?
3. “Elements with shells that are already full and have no electrons to lend are called noble gases—
and helium, the smallest of these, is considered the most inert.”
What is the meaning of the word “inert” in the sentence above?
4. Why don’t helium and the other noble gases normally make compounds?
5.
Why wouldn’t helium take, give or share electrons when making this weird compound?