By Ron B. Davis Jr., Georgetown University
Hydrogen has one proton and one electron in its most common isotope, making it the simplest element in the universe—the mother (or father) of all other elements. Today, the periodic table is practically always presented in same form, with hydrogen in row 1, in group 1 on the far left.
Hydrogen and Alkali Metals
Hydrogen exhibits behavior that could potentially earn it a place in at least two other locations in row one of the table.
On the one hand, hydrogen has similarities to lithium, sodium, potassium, and the other alkali metals in group 1. Hydrogen’s 1s1 ground-state electron configuration makes it analogous to the alkali metals in group 1.
Hydrogen can, and sometimes does, lose its only electron to form an ion with a plus-one charge in a behavior that mirrors that of the alkali metals.
This plus-one charge on a hydrogen proton is what makes many hydrogen-containing compounds acidic. Compounds that deliver hydrogen ions into solutions, like hydrochloric acid, for example, reduce a solution’s pH when added.
Chemists measure the acidity of solutions using the ‘pH’ scale, with lower numbers on the scale indicating more acidic solutions. The ‘H’ in ‘pH’ refers to hydrogen ions—which is just the single proton, without the electron. And it’s these H-plus ions—these protons—that give acids many of their signature properties, like the ability to dissolve certain metals.
But all of the other group 1 elements are metals, so if hydrogen belongs in group 1, shouldn’t it be metallic?
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Metallic Hydrogen
Astronomers took up this question as far back as 1935. That was when astronomers, trying to understand huge gas-giant planets like Jupiter, first calculated that hydrogen might behave like a metal under the intense pressures in the core of huge gas giants like Jupiter.
But it wasn’t until 2016 that a team at Harvard apparently created metallic hydrogen in the lab for the first time. The lab used extreme pressure, more than 4 million times greater than that on the surface of the Earth. Meanwhile, the temperature was kept super cooled to just 5.5° above absolute zero.
Under these conditions, it appears as though hydrogen’s 1s electrons are relinquished to a valence shell where metallic bonding takes place. This creates a metal with a lustrous appearance and electrical conductivity consistent with a metallic state.
So, hydrogen sure does seem to have many properties of a group 1 alkali metal.
Hydrogen and Group 17 Elements
Yet, on the other hand, having a 1s1 electron configuration also means that hydrogen is just one electron shy of a full first valence shell. This makes hydrogen more akin to a group 17 halogen in certain ways.
Though much less common than losing an electron, hydrogen can sometimes accept an electron and form an ionic bond like the group 17 elements. For example, sodium plus hydrogen forms sodium hydride; a solid, ionic material sometimes used as a means of storing hydrogen for use in fuel cells.
Unlike in the cores of gas giants or the extreme conditions created in laboratories, under conditions that we experience here on Earth, elemental hydrogen commonly forms a diatomic molecule joined by a covalent bond that completes its valence shell, just like fluorine and the other halogens do.
In these ways, taking on electrons to form an anion and bonding in pairs to form a diatomic molecule, hydrogen sure does seem to be more like a halogen.
To make things even more interesting, hydrogen sometimes doesn’t seem to fit into either of these groups!
Hydrogen’s Electronegativity
If we look at electronegativity as the third dimension for elements on the top two rows of the table, we can see that hydrogen looks entirely out of place on the far left. But it also looks entirely out of place on the far right. In fact, hydrogen’s electronegativity looks awfully similar to carbon, right in the middle of the row!
Well, we can think of hydrogen as having a half-filled outer shell (1 of 2 electrons), just as carbon also has a half-filled outer shell, with 4 of 8 electrons. This similarity gives hydrogen and carbon similar electronegativities.
Perhaps most interestingly, having exactly half of their valence shell electrons means that when hydrogen and carbon are fully saturated with chemical bonds, their entire valence shell—every last electron—participates in bonding. Hydrogen and the group 14 elements are the only elements able to accomplish this. So there are some legitimate reasons to consider placing hydrogen above carbon on the table, as well.
Periodic Tables of Mendeleev and Benfey
Although there is certainly room for debate, Dmitri Mendeleev’s table places hydrogen in group 1. Based on one of the few properties Mendeleev had to work with, hydrogen does indeed behave like an alkali metal, combining in a two-to-one ratio with oxygen—water’s molecular formula, H2O is consistent with the oxides of alkali metals, like Li2O, Na2O and K2O, for example.
And although its chameleon-like behavior is now better understood, these days hydrogen almost always takes its place in column 1 of the periodic table, although some versions of the table through the ages have, in fact, placed hydrogen above the halogens in column 17.
And, there is one other type of table that offers to settle the debate in a different way, by creating a uniquely special place for hydrogen. This is the spiral periodic table, proposed by Theodore Benfey in the early 1960s. In Benfey’s table, the element hydrogen takes on what many might consider to be its rightful place—dead at the center of the spiral—where its similarities to the group 1 alkali metals, the group 14 elements, and the group 17 halogens are all given equal footing.
Common Questions about Element Number 1, Hydrogen
Hydrogen’s 1s1 ground-state electron configuration makes it analogous to the alkali metals in group 1.
The ‘H’ in ‘pH’ refers to hydrogen ions—which is just the single proton, without the electron.
In 2016, a team at Harvard apparently created metallic hydrogen in the lab for the first time. The lab used extreme pressure, more than 4 million times greater than that on the surface of the Earth. Meanwhile, the temperature was kept super cooled to just 5.5° above absolute zero.
