By Ron B. Davis Jr., Georgetown University
Row one of the periodic table is inhabited by just two elements—hydrogen and helium. These two were among the first elements created during the birth of our universe. They are the source material for the formation of nearly all other elements, and they are the most abundant elements in our universe even today.
Reactivity and Availability
As similar as they are in size, in most other ways helium and hydrogen couldn’t be much more different than one another.
Hydrogen’s relentless quest to acquire a second electron makes it highly reactive as a lone atom. Even when bonded to another atom of hydrogen, the resulting H2 molecule is still reactive enough to provide the chemical potential energy that can be burned to power rocket ships.
In contrast, with a closed valence shell in full view of its nucleus, helium is the single least-reactive element on the entire table.
So, unlike hydrogen, which can be extracted from compounds like methane, helium doesn’t combine chemically with other elements, making it truly scarce on Earth.
This explains its name, which comes from the Greek word helios—an homage to its discovery in the outer reaches of the Sun’s corona—two decades before it was ever isolated here on Earth.
Helium: Byproduct of Radioactive Decay
All the helium we have here on Earth is a byproduct of radioactive decay from much larger atoms in rocks. Since the formation of the Earth, primordial radioactive elements like uranium and thorium have been undergoing their decay chains, which include alpha decays that slowly release helium nuclei underground.
If these helium nuclei are trapped within the rock ore that produced it, they can acquire two electrons to become helium atoms. Over millennia, these helium atoms can slowly leak out of the mineral, sometimes becoming trapped in pockets alongside natural gas, which is where the vast majority of the world’s helium supply is sourced.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Presence of Hydrogen on Earth
When hydrogen is in its common elemental form, two hydrogen atoms bond together through a single covalent bond to obtain a full valence shell. In this form, hydrogen is a gas.
Hydrogen gas is only about one-tenth as dense as the air around us. So, any hydrogen gas that we might create or collect here on Earth must be trapped immediately in a container, or it will simply drift into space. It’s easy to see why the Goldschmidt system classifies hydrogen as an ‘atmophile.’
And yet, hydrogen is still very common on Earth. The catch is that it exists naturally only in compounds with other elements.
Differences in Properties
Molecular hydrogen has properties similar to group 17 halogens under standard conditions where it appears as a diatomic molecule and can even form a hydride anion. But hydrogen also acts more like a group I alkali metal when dissolved in water to form acidic H-plus ions, or under conditions of extreme pressure where it takes on metallic characteristics.
Helium’s s2 configuration might give it guest privileges at the top of group 2 in the table. But helium’s unrivaled lack of reactivity and its closed valence shell make for a clear permanent home atop the noble gases of group 18. All of helium’s properties are consistent with those of the noble gasses.
Hydrogen’s tendency to form a single bond makes it a diatomic molecule as a pure element on its own under ordinary conditions, but hydrogen’s willingness to trade those bonds with other elements like oxygen to form water and carbon to form methane ensures that there is plenty of it in our environment for us to chemically extract for a number of uses.
In comparison, helium’s remarkable stability and buoyancy give it a very different set of highly-valued properties, while also explaining why helium is vanishingly rare in the world around us.
Uses of Helium
Helium is most familiar from the filling of balloons to give them buoyancy and make them rise. Whether in a simple party balloon or a sophisticated weather balloon, helium is often the gas of choice to give buoyancy without the dangers of explosion that hydrogen produces.
But helium is useful for more than just aviation and party decorations.
Liquified helium boils at just 4 Kelvin, very close to absolute zero, also making it the coldest readily available liquid in the world. Its inertness, combined with its remarkably low temperature, makes liquid helium valuable for the demanding conditions required to explore extreme states of matter, or to operate superconducting magnets, like those employed in magnetic resonance imaging.
Using Hydrogen in Nuclear Reactions
On November 1, 1952, the United Stated detonated an explosive device code-named Ivy-Mike on a small pacific island called Elugelab.
Ivy Mike was the world’s first ‘hydrogen bomb’—a device that carried out a nuclear reaction, fusing hydrogen into helium and releasing the resulting energy in an instant, creating the most powerful explosion ever created by mankind—almost a thousand times more powerful than the nuclear bomb that destroyed Hiroshima.
However, the hydrogen used in so-called H-bombs isn’t the most common isotope of hydrogen consisting of just a proton in its nucleus. The reaction is driven by the formation of the very stable helium-4 nucleus. But when using the typical hydrogen nucleus as the fuel, something doesn’t add up. In order to get the fusion reaction to work as reliably and cleanly as possible, its designers used a special isotope of hydrogen, called deuterium, which also contains a neutron.
Deuterium and Tritium
In nature, deuterium isn’t very abundant, but it’s there.
Using deuterium essentially primes two hydrogen nuclei with two neutrons so they can fuse into the much more stable helium-4.
In fact, modern hydrogen bombs use even more sophisticated fusion reactions involving tritium, an isotope of hydrogen having two neutrons in its nucleus. Tritium is used to enhance the explosive yields of nuclear bombs, because fusion between a tritium and a deuterium atom forms not just a helium nucleus, but also releases a spare neutron. This extra neutron can then go on to trigger other nuclear reactions, releasing even more energy during a detonation.
Common Questions about Helium and Hydrogen
The term helium comes from the Greek word helios—an homage to its discovery in the outer reaches of the Sun’s corona—two decades before it was ever isolated here on Earth.
Helium is often the gas of choice to give buoyancy without the dangers of explosion that hydrogen produces.
Tritium is an isotope of hydrogen having two neutrons in its nucleus. It is used to enhance the explosive yields of nuclear bombs.
