What's So Special About Gold, Anyway?

Transcript

ERIC: From the Museum of Science in Boston, this is Pulsar - a podcast where we prospect for answers to the most auriferous questions we've ever gotten from our visitors. I'm your host, Eric. And sometimes we get questions that are less specific and more existential. Like, why is gold so special? The best part about these questions is that they usually have quite a few different answers. Joining me to discuss all of the amazing chemistry that makes gold unique is Colin from our education team. Colin, thanks for joining me.

COLIN: Hello. It's good to be here.

ERIC: Why don't we start with where gold comes from. What is gold's origin story?

COLIN: Yeah, that's a really good question. I find it so romantic to look at the periodic table. And to sort of understand that, you know, at the very beginning, it was really just energy, right. And then as that energy cooled and coalesced, it became atoms - hydrogen atoms and helium atoms, basically the lightest elements in the universe. And then as those atoms began to experience gravity, they formed stars, and then eventually, those stars exploded. And that is actually how we get all of the heavy elements due to nuclear reactions due to all kinds of things that we're actually still coming to understand. Because we are still looking into the cosmos to better understand how this nuclear physics works. But from our understanding, almost all of the elements after iron come from stars exploding, or stars colliding, and all of these really energetic, wonderful things that happen when stars get old, and stars interact with each other.

ERIC: Yeah, we started out with just hydrogen, maybe a little bit of helium in the universe. And stars are element factories. So everything in the periodic table is made by those stars, smashing the hydrogen and the helium together. And eventually smashing the helium together into everything on up to iron. And then even stars in their cores, millions of degrees, just not enough to push past that and fuse two iron atoms together. And so it takes a star literally ripping itself apart and a huge explosion to get those conditions for a split second to create anything above that, including plutonium, uranium, all these exotic things, and gold.

COLIN: Yeah, and that is why you find that all of those elements after iron are so rare in the universe, rare on Earth and rare in the universe as a whole. There's just not that much of it, because it requires those special conditions to make it.

ERIC: Yeah, you could find calcium and silicon just kind of around, all over the place, in our rocks. Oxygen is obviously all over the place, hydrogen in our water, H20. But once you get up past iron, there's just not a lot of it around because those events are pretty rare. I mean, stars are making those elements all the time. They're element factories, but the ones that do explode and have the conditions for that are exceedingly rare, you have to have a massive star to actually explode.

COLIN: Yeah, absolutely.

ERIC: And I'm glad you mentioned the collisions, because that's something that I didn't realize until recently. I had always said, oh, it comes from exploding stars. But it's so cool that it can actually come from two stars smashing into each other, which is almost cooler than exploding.

COLIN: And also put it into perspective, given how long the universe has been around, that all of these elements can be found on Earth, right? It's probably taken a lot of these collisions and a lot of these explosions for all of these heavy elements to actually end up in the universe and eventually coalesce onto our planet.

ERIC: Yeah, a couple of different life cycles of stars, forming, making these elements exploding, seeding that stuff on the universe, and then you just have like, gold atoms floating around. And they did become part of the next star systems that formed. But you have to build that up over a couple of those cycles. It takes a couple billion years, right?

COLIN: Yeah, absolutely. I do think it's really fascinating. Most of the gold that is on the earth, we will never be able to access. It turns out that most of the gold is actually in the core. And so the gold that we can access is the gold that is solid, you know, found in rocks found on the surface of our planet. But the vast majority of it is actually in our core.

ERIC: How far down under the earth can humans go before it gets intolerable? Before we've reached limits of our technology?

COLIN: Yeah, so the current record is seven kilometers, I believe that's around for three or four miles. We have not been able to dig very deep. And the main reason for that is that it gets really hot, really fast, and our equipment breaks down. And we stopp being able to move the rock that's in our way. So all that gold that's beneath the surface, it's pretty much untouchable as far as our current technology goes.

ERIC: So that's one reason why gold is so special, because it's super rare and has this awesome origin story. But it's also special because of its properties. Gold is a metal and has properties that are kind of unique among all materials. So can you talk a little bit about what makes it so special, in terms of working with gold?

COLIN: Gold is really, really soft. In science terms, we call that ductile, or the ability to sort of pull it into a wire. It's also malleable, that is the ability to sort of squish it down into a very flat surface. Gold is so easy to manipulate. And because gold is conductive, it's able to transfer electrons very easily. We use it in all kinds of electronics, we use it in our phones, and basically all these different devices that we have, because of these really special properties.

ERIC: It's sort of weird that it's a metal. And we usually think of metals as things that are conductive, which a lot of them are, but also super hard and strong. And gold is actually soft. Like you wouldn't want to make a hammer or a sword out of gold. So why is it so soft?

COLIN: Yeah, that has to do with the structure of the electrons on the outside. If you've ever heard of metallic bonds, generally, when metals are bonded together, they have what is called a sea of electrons that sort of allow for the conductivity, but also allow for the structural stability of those atoms being in place. Basically, gold atoms are able to slip by each other really, really easily because of the unique electron sea properties of the metal itself. There are not very metals that act like this. For example, iron does not act like this, those atoms are not able to slip past each other. But with gold, we are able to make gold sheets that are essentially atoms thick, because of the fact that they're so movable. They're so mobile. In the early 1900s, there was this very famous science experiment called the Rutherford gold foil experiment, where some scientists essentially shot a beam of particles at really, really, really thin gold foil. And in the early 1900s, they were basically trying to figure out what atoms look like, what the structure of an atom is. And by doing this experiment, they basically struck this gold foil with a beam of particles and saw that the gold foil actually allowed for most of those particles to pass straight through it, they were able to discover that atoms are mostly empty space. That was not something that was known 100 years ago. And because of gold, because of how manipulatable those atoms are, we were able to see that gold is mostly empty space, allowing these particles to pass right through.

ERIC: It makes me think about how cool the periodic table is where we have these elements. And the only difference from one to the next one is how many protons are in the nucleus. Things that are next to each other that just have one more proton in the nucleus can behave completely differently.

COLIN: Yeah, it's really, really fascinating.

ERIC: So gold is...gold. And that is not just a substance, but a color. Is it also the number of protons and its structure that gives it its color?

COLIN: In this case, it's not the protons actually, it's the number of electrons on the outside. Gold generally, when you find it as a metal, has an equal number of electrons and an equal number of protons. And it's the structure of the electrons on the outside that allows certain wavelengths or certain energies of light to bounce off of it, and absorb some of those wavelengths of light. Gold has what we call an absorbance band, or a range of light that it absorbs. And because it absorbs blue light, specifically, it actually has this very characteristic yellow color, this golden color.

ERIC: So it absorbs blue light, but it also reflects a lot of light really, really well. And that's why it's used in order to reflect things, like in even the inside of some telescopes.

COLIN: Absolutely. And one of the most famous telescopes right now is of course, the James Webb Telescope. We have a telescope out in space right now that is covered in gold. And that is because this telescope is not looking in the visible light spectrum that you and I see in. It is looking in the infrared spectrum. And while gold is known for being yellow, is known for absorbing in the visible range, it does not absorb at all in the infrared range and is a really good mirror for this purpose.

ERIC: And it's been really cool to talk to our visitors here about the fact that the Webb telescope doesn't look like a big mirror. When you look at pictures of the inside of Hubble, you see that big bowl shaped mirror and there's pictures of the scientists looking in and seeing themselves. And for Webb, it's just a bunch of, you know, big gold honeycomb things. And people are asking, like, wait, that's a telescope? How does it work? Is it a weird camera? And it turns out that no, in the infrared, if our eyes could see infrared, that would look like a mirror to us because it's so good at bouncing infrared light.

COLIN: Yeah, it is also so fun when the best scientific tool also turns out to be one of the most beautiful things that people can see. Just seeing that those 18 hexagons of gold is also just awe inspiring. So it's really cool that gold both has this awe inspiring effect on us as viewers, but also has this really key scientific property of reflecting that infrared light and allowing the Webb telescope to see.

ERIC: Those gold atoms on the Webb telescope have like maybe the most amazing journey ever, starting out in an exploding star and then becoming a part of a whole planet and then being dug up by humans and stuck on a telescope and launched out to understand, you know, the things that made it in the first place - those exploding stars and the rest of the universe.

COLIN: Absolutely. And again, to get back to that point of gold being easy to work with. There's not actually that much gold on the Webb telescope. It is a very thin film, basically, that just covers the very outside.

ERIC: Well hopefully the next time our listeners see a breathtaking image from the James Webb Space Telescope, they'll remember that it was only possible because gold is so special. Colin, thanks for joining me.

COLIN: My pleasure. It was great to be here.

ERIC: The next time you visit the Museum of Science, take a stroll along our Rock Walk to explore the amazing properties of other materials that make up the Earth. And from home you can visit the Rock Walk virtually to explore how scientists know how old each rock is in our Sparks of Science video series. Until next time, keep asking questions.

If you liked this episode, be sure to check out:

What Is Your Favorite Molecule?

How Can I Explore Chemistry at Home?

Why Do Stars Explode?

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