The Coolest Thing You Likely Never Learned About Hydrogen

What do you know about Hydrogen? In a very un-scientific study, I decided to find out. I posed this question to my Facebook friends: “If I ask you to tell me 1 thing you know off the top of your head about Hydrogen, what would it be? The cooler the factoid the better.” I have a bunch of science and non-science Facebook friends, and got a nice spread of answers. Here’s some of them, more common ones closer the top (can you guess which answers were from science majors?):
1) It’s in water (H2O)
2) Most abundant element in the Universe
3) It’s highly flammable/explosive in air
4) It’s a proton
5) Nuclear fusion (and fusion bombs)
6) 1st on the periodic table
7) The only wavefunction we can solve exactly
8) Hydrogen bonds with certain elements strongly
9) Hydrogen in hydrocarbons when burned release greenhouse gases
10) Dr. Manhattan’s logo
11) Proton pump in mitochondria
12) H+ ions in acids
(Many more great answers, too many to include them all!)

I think these results paint a great picture of why Hydrogen is really cool, and really important. It also provides strong evidence for the assertion I put as the title of this post. Even amongst my science/astronomy friends, nobody mentioned what I think of as the coolest factoid about Hydrogen (but granted, I’m a nerdy astronomy student and I only asked for one factoid). It’s especially important to the understanding of stars and what we see when we look at the sun! I’m talking of course about the Hydrogen anion (H-). That’s right…you can have negative hydrogen, and its properties are really cool!

To understand negative hydrogen, let’s talk about some chemistry ideas. First, recall the Bohr model of the neutral hydrogen atom: you have a proton nucleus, with an electron zooming around in specific energy levels. The electron is bound to the proton because their opposite charges make them attractive…the proton holds onto it through this electrical tugging. Though this model is too basic and not exactly correct, it helps us intuitively illustrate physics fundamental to our understanding of the Universe—that when an electron jumps (transitions) between two different energy levels, light gets either absorbed (lower level to higher level) or emitted (higher level to lower level). If both of these energy levels keep the electron bound to the nucleus, we call it a bound-bound transition. Neutral hydrogen gas has many bound levels, and therefore has many bound-bound transitions (some of them corresponding to visible colors)! If the electron absorbs light with lots of energy, the electron escapes the atom completely—this is called a bound-free transition, since the electron ends up free of the tyrannical proton nucleus…forming an H+ ion!

Let there be light!

But there’s another, more wacky form of Hydrogen! Hydrogen is highly polarized, and can actually capture an additional electron, forming a negative ion. As you might expect, this ion is very weakly bound together—in fact, it only has one bound state (the ground state), and the 2nd electron can easily escape through absorption of light! Since it only has one bound state, it is impossible for H-, also known as hydride, to have bound-bound transitions. However, since it is easy to ionize (eject it’s second electron), it readily absorbs light, especially in near-infrared, visible, and UV wavelengths (and even higher energies too!).

Hydride

For those of you with a scientific fascination with awesome atomic configurations, I highly highly encourage you to delve deeper into the characteristics of this fascinating ion, but I want to focus on the reason it’s really cool/important to me (as an astronomy student). One of the places hydride shows up is in the atmosphere of the sun! The Sun is a giant ball of hot plasma—its photosphere (where most of the light escapes the sun’s surface) is around 6000 degrees Celsius—and it’s even hotter as you get deeper and deeper in. This plasma makes it difficult for light to escape the surface—it is constantly interacting with the protons and electrons, getting absorbed and scattered all over the place. In fact, despite the incredible speed of light, it can take a photon of light up to 100,000 years or more to escape! The difficulty of light to escape from a gas or plasma (like the Sun) is called it’s opacity. When something has a high opacity (more opaque), you cannot see deeper into it.

The Sun seems to have a surface—the photosphere—under which the Sun becomes very opaque. It is very hard for radiation from deeper in its atmosphere to escape…and the reason? You guessed it—the negative hydrogen ion! Hydride readily forms in the Sun’s atmosphere, so there’s a lot of it. There’s also a lot of light energy trying to escape the Sun, which encounters the H- ions. Because it’s so easy to ionize hydride, the hydride will absorb the light and the second electron escapes. This happens so much that the light has trouble punching through all the hydride ions, which keep forming to continue blocking this light. Those pesky hydride ions are therefore the major source of the Sun’s opacity! And this is not just true for the Sun, but for many many other stars as well!

Go home, photon: you're drunk!

Go home, photon: you’re drunk!

So that’s my favorite thing about Hydrogen—it’s negative ion! Was it something you learned in school? What’s your favorite factoid about Hydrogen (that you can name off the top of your head)? Leave a comment! I want to once again thank all of my wonderful friends for their great responses to my Hydrogen question—it’s amazing to see how many answers I received from different people, and the support of a somewhat silly venture of mine is always something I appreciate. Until next time, friends!

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4 Responses to The Coolest Thing You Likely Never Learned About Hydrogen

  1. Vivienne says:

    One other cool thing about H-: it enabled the formation of molecular hydrogen in the early universe, allowing the first stars to be born (stars form from molecular hydrogen)! Today, molecular hydrogen can form on interstellar dust grains, which is a much more efficient way to make it. However, in the early universe, there really wasn’t any dust yet, so the molecular hydrogen for stars had to form from H- and H+ joining together.

  2. Brian Kateman says:

    Stellar post, with emphasis on, stellar. What would happen if H- was not present in the sun? Would we’d fry?

    • That’s a great question—I’m not 100% sure, but I consulted the fellow grad students in my office and I’ll give you our decided answer. So I talked about how H- creates opacity in the Sun’s atmosphere because it absorbs light to ionize, but it also emits light during it’s formation (the capture of the second electron creates an emission at one specific energy). If there’s a steady state—just as many created as destroyed—then getting rid of the H- wouldn’t change the outward brightness of the Sun, so the Earth’s temperature would not be affected. If however, there isn’t a steady state, then whatever energy is getting stored or generated in the creation or destruction of H- would go away if removed, and then there would be a temperature change here at Earth. To our knowledge, there is a steady state, so we agreed that we think there’s be no brightness (and therefore temperature) change here. However, what would happen for sure is that if you pointed a telescope at the Sun, if H- was not present than you would be able to see to greater depth inside the Sun (more transparency).

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