By Ron B. Davis Jr., Georgetown University
By the first decade of the 1900s, it was becoming clear that hydrogen is the simplest and lightest element in the universe. It is also the most abundant element in the universe, making up about 74% of all observable matter. On the other hand, helium doesn’t combine chemically with other elements, making it truly scarce on Earth.
Separation of Pure Hydrogen
The first successful separation of pure hydrogen happened at the hands of Robert Boyle in 1671. Boyle discovered that reacting iron filings with dilute acid produces a gas that could be burned in air to form water.
About a century later, French chemist Antoine Lavoisier recognized that the gas formed in Boyle’s experiment was, in fact, an element. And it was Lavoisier who coined the name ‘hydro-GEN,’ in recognition of its ability to ‘GEN-erate’ water when burned in air.
These days, the most common technique to obtain hydrogen involves a partial combustion of the natural gas molecule called methane.
Hydrogen atoms in methane are bonded to carbon, forming a small molecule with the formula CH4. That’s just four hydrogen atoms and a carbon atom. Methane forms a gas that can be completely combusted in air to form carbon dioxide and water. But by burning methane with a carefully controlled amount of oxygen, a situation can be set up in which there is only enough oxygen to combine with the carbon, leaving the hydrogen atoms to become molecules of hydrogen gas, instead of water as they do in complete combustion.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Hydrogen in Water
Hydrogen also makes up two thirds of the atoms in water—the most abundant chemical compound on the surface of our planet. The reason that we see so much hydrogen tied up in the form of water is that a water molecule is far more stable than either oxygen gas or hydrogen gas alone. So, when given the opportunity to combine, hydrogen and oxygen join in a chemical reaction that creates a more stable water molecule and releases a tremendous amount of energy.
Today we use this chemical reaction between hydrogen and oxygen as a source of energy to power some our most advanced vehicles, such as rocket ships and the space shuttle.
But we get even more power from hydrogen in the form of nuclear reactions.
Structure and Position of Helium
Element number 2, helium, has two protons, and two electrons, but also two neutrons in its most common isotope. These neutrons help to dilute the positive nuclear charges of its two protons, helping the nucleus to stay together.
The number two is magically stable for nuclei, and the result is that helium’s nucleus is remarkably stable compared to other light elements. This makes helium the stepping-stone of stellar nucleosynthesis, serving as an intermediate between hydrogen and many of the other, larger elements around us.
In the case of helium, there’s not much debate about its position. Still, there is some ambiguity caused by its very small valence shell, which can only hold two electrons at most.
Helium’s electron configuration is 1s2. So, one might argue, helium could be atop group 2 in the ‘s-block’ of the table, above beryllium, magnesium, and calcium, all of which have an ‘s2’ configuration in their valence shell as well.
Charles Janet’s ‘Left-step’ Table
Taking this idea further, in 1928, shortly after the discovery of subshells, Charles Janet proposed what is called the ‘left-step’ table. This arrangement is unfailingly loyal to electron configurations. Helium is above beryllium instead of above the other noble gases, but even more adjustments are made.
By relocating the two s-block columns from the far left to the far right and moving the two bottom rows up to the left, we produce an extended layout. Just as with the more common form of the table, this layout reads from left to right and top to bottom like a paragraph, still taking us through the progression of orbitals in the aufbau order.
However, this arrangement fails to give the noble gases a special position at the end of each row. This was one reason, Janet’s creation—though technically sound—hasn’t been widely adopted by the chemistry community.
Inertness of Helium
The second reason is that scientists have not been able to observe, or even imagine, any theoretical conditions under which helium might ever behave like an alkaline earth metal.
With its closed first valence shell, helium is the undisputed king of chemical inertness. Of all the elements on the table, it is the least willing to give up electrons to form ions or metallic bonding, in contrast to the group-two metals, which readily do both.
So, there’s really no ambiguity about grouping helium based on its properties: Helium at the top of the noble gases is clearly the best choice, due to its unrivaled inertness and its closed valence shell.
Common Questions about Hydrogen and Helium: The Two Elements in Row 1 of the Periodic Table
In 1671, Robert Boyle successfully separated pure hydrogen. Boyle discovered that reacting iron filings with dilute acid produces a gas that could be burned in air to form water. About a century later, French chemist Antoine Lavoisier recognized that the gas formed in Boyle’s experiment was, in fact, an element. And it was Lavoisier who coined the name ‘hydro-GEN,’ in recognition of its ability to ‘GEN-erate’ water when burned in air.
These days, the most common technique to obtain hydrogen involves a partial combustion of the natural-gas molecule called methane.
Charles Janet’s arrangement fails to give the noble gases a special position at the end of each row. This is one reason, Janet’s creation—though technically sound—hasn’t been widely adopted by the chemistry community.