Specific properties of hydrogen have led to an assumption way back in the past that it is probably metal. Its atom, for example, has an electron configuration just like alkali metals. However, this chemical element is not a metal.
This is a liquid hydrogen fuel tank used in SLS rocket. Image credit: NASA
There are 118 elements known to humans – based on their properties, scientists arrange all elements in the periodic table, grouping them into categories that we call metals, non-metals, and metalloids.
Hydrogen stands in this table right above lithium, which is an alkali metal, and both of them have their outermost electron in an s-orbital. But the electron configuration is not the only property separating metals from non-metals. So why is hydrogen placed in the alkali metal group in the periodic table of chemical elements?
Differences between hydrogen and alkali metals
At the time when this table of chemical elements was created, it just seemed like the correct place to put the hydrogen in. After all, it shares the same basic structure of electron orbitals, known as ns1.
It also has other similar properties – hydrogen, for example, can react with mercury in combination with nitrogen, forming amalgam, like most metals. This implied similarity to alkali methods is the main and probably the only reason why hydrogen appears in the same group as lithium, sodium, potassium, rubidium, caesium, and francium.
This is the periodic table of chemical elements, showing theoretical distributions of their electron orbitals. Hydrogen is the first in this table, and also the most common substance in the Universe. Image credit: Visualizing All Things Science via Flickr, CC BY-NC 2.0
But if we look into the chemical properties of this element, it behaves differently from alkali metals.
For example, hydrogen’s ionization energy is 1312 kJ/mol, while the alkali element with the highest ionization energy – lithium – needs just 520 kJ/mol to become ionized, i.e. in order to remove an electron from its atom.
Why isn’t hydrogen a halogen?
Hydrogen can also form hydride anions – negative ions – such as LiH. Because of this property, it belongs to nonmetals. Sometimes it is even placed above the halogens in the periodic table of chemical elements.
Is it a halogen then?
Well, no. It can form dihydrogen H2, similarly to elements from the halogen group, but its electron affinity is lower. This means that hydrogen is considerably less likely to gain an additional electron and the amount of energy released during this process is much smaller.
Metallic hydrogen – strange phase of matter
Interestingly, under certain extreme conditions – under strong pressure and high temperature – hydrogen atoms can make a transition to a so-called “degenerate” state where it enters a metallic phase and starts to behave like an electrical conductor.
This is an example of the so-called diamond anvil cell (DAC) that is used to achieve pressures comparable to those in the center of a planet. Image credit: Harvard University
We don’t have such a form of the matter here on Earth, but it is thought to exist in big amounts inside the largest planets, such as Jupiter and Saturn.
How difficult is it to make metallic hydrogen? In the world, there are only very few places where these kinds of experiments could be done.
To force hydrogen atoms to enter the metallic phase, you would need a pressure of about 495 gigapascals or more. This is nearly 5 million times more compared to standard atmospheric pressure here on Earth. At such hellish conditions, any regular matter behaves very differently compared to what we are used to.
According to a lab experiment performed at Harvard University in 2016, hydrogen solidifies at 495 gigapascals and becomes black and reflective.
In this diagram you can see a theoretically predicted internal structure of Jupiter – notice that metallic hydrogen is one of the major constituents of giant planets. Image credit: Kelvinsong via Wikimedia, CC BY-SA 3.0
Wrapping up
Hydrogen is the basic chemical element that makes up the largest part of the universe. At the same time, it has a lot of unexplored and quite mysterious properties. Due to its unusual behavior, it is not possible to assign it uniquely to metals or non-metals. However, under extreme pressure, hydrogen transforms into a solid metallic phase which is thought to exist inside giant planets such as Jupiter.