It may be toxic, but mercury sure looks magical.

So much so that, according to legend,  the ancient Chinese emperor Qin Shi Huang drank it thinking it would give him eternal life. A bit misguided?

Sure. But honestly? Understandable.

After all, it’s a metal that manages  to be a liquid at room temperature… even if that room has a window  open and it’s snowing outside. I’m not exaggerating. Mercury doesn’t freeze until it  drops below -39 degrees Celsius!

But as visions of mercury blobs  skitter through your mind, you may be wondering why mercury is so special. Well, in order to answer that  question, we have to turn to a theory you might associate with space  travel more than chemistry. A theory that taught us that  both time and space are relative, and that E = mc².

That’s right. Mercury is a liquid because of Special Relativity. [♪ INTRO] Let’s start our exploration of all this  weirdness in the shallow end of the pool… which I’ve filled with water, not mercury…and  recall a bit of high school chemistry. The periodic table isn’t just a  cool thing to hang on your wall.

It’s a top-notch organizational device. Because elements that share  a column with each other usually share some of the same properties. For example, everything in the leftmost  column of the periodic table is very reactive, meaning the atoms really want to form  strong bonds with one or more other atoms.

In contrast, everything in the rightmost  column is the opposite of reactive. You know, the noble gases… And finally, there’s everything in the middle. Which mostly comes in shades of…sorta reactive.

Mercury, you’ll notice, is one of the many,  many elements in the middle of the table. It belongs to a group of metals  known as the transition metals. And it’s more specifically a member  of Group 12, along with zinc, cadmium, and the only-exists-when-humans-make-it  copernicium.

But wherever you are on the table,  your reactivity is generally determined by how many electrons you have, and how  they’re arranged around the nucleus. Now, you might remember being told  to picture this arrangement like so, where the electrons sit in a  series of concentric circles. This setup is sometimes  known as the planetary model, because it vaguely resembles  planets orbiting the Sun.

And it’s…well… it’s wrong. Because there’s this fun thing that happens when you shrink down to subatomic sizes. Electrons are revealed to not  be teeny, discrete particles, governed by the traditional laws of physics.

They’re simultaneously both particles and waves, obeying the fuzzy laws of quantum mechanics. So while electrons do orbit nuclei,  it’s not like how planets orbit the Sun. But if we acknowledge it’s  wrong, we can take advantage of this model’s simplicity and use it  to understand why mercury is a liquid.

Can we all agree on that? Ok, cool. Electrons orbit an atom’s nucleus  in specific locations called shells.

Each shell can hold a different number  of fun-looking shapes called subshells. Shell 1 can hold one type of subshell,  Shell 2 can hold two, and so on. We don’t need to get into all the nuances  for this video, but these subshells get more and more complicated as you go,  creating more electron storage spots.

For example, an s subshell has two  spots, and a d subshell has ten spots. So the higher the shell number, the  more electrons you can pack into it. And as you learned in chem class, all these shells and subshells tend  to fill up in a very specific order.

But I am a benevolent host, so we are going to skip right to the  arrangement for mercury and its 80 electrons. Voilà. I know it looks a bit complicated,  but the most important thing to notice is that all of the subshells that have  any electrons are completely full.

Compare this to all of the other transition  metals in mercury’s row, which have at least one electron missing from either their  6s subshell, their 5d subshell, or both. You see a similar feature  in other Group 12 elements. They don’t have as many electrons to deal with, but their outermost s and  d subshells are full, too.

And in chemistry, a set of completely  filled subshells makes an atom very happy… as much as a non-sentient particle  can express a human emotion. It’s more correct to say that  the atom is less reactive. But since that doesn’t come naturally  to most elements on the periodic table, the typical way for an atom  to do this is by making bonds.

When you’re in a chemistry class,  you usually think of that happening in terms of a chemical reaction. Like putting a bunch of  “please-take-this-electron” sodium and “please-gimme-an-electron”  chlorine together to make table salt. But those aren’t the only  kinds of bonds atoms can make.

When you’ve just got a bunch of the  same atoms chilling out together, like a lump of pure sodium, or a pool of  mercury, you’re working with metallic bonds. And under the right conditions,  like if you crank up the thermostat, you can loosen those bonds and change  a solid lump into a liquid pool. Since the Group 12 elements all have  that filled outer s and d subshell combo, the metallic bonds that their  atoms do form will be weaker than those made by the rest of  their transition metal cousins.

Which means they’ll have  noticeably lower melting points. For example, zinc melts at  about 420 degrees celsius. While that’s scorching to you or me,  it’s downright chilly compared to copper next door, rocking a melting point  that’s over a thousand degrees.

Cadmium, meanwhile, melts a  bit sooner…at 320 degrees. But mercury, as I stated in  the intro to this episode, will be a liquid as soon as it’s  warmer than -39 degrees Celsius. It melts before water does!

As it turns out, mercury atoms  are so uninterested in bonding that they mostly hold themselves together with a kind of electrostatic  attraction called Van der Waals forces. These forces are a lot weaker than your  standard bonds, and arise from the fact that at any given moment, an atom’s  electrons aren’t evenly distributed. In other words, you have regions where there’s a tiny amount of extra negative charge.

And those regions can attract regions of  the next atom over where the electrons don’t happen to be, making them  a little more positive in charge. Now, one reason why mercury is so reliant on these weak Van der Waals connections is  because, unlike zinc and cadmium, it’s big enough to put some of  its electrons into an f subshell. And if you thought d subshells looked  complicated, f subshells are even more so.

They can have all sorts of  weird effects on an atom, including metallic bonds that are  weaker than they otherwise “should” be. Unfortunately, this still doesn’t explain why  mercury’s melting point is as low as it is. That’s right.

It’s time for special relativity to step in. This was Albert Einstein’s first version of  relativity, and it completely reinvisioned the relationships between space  and time, and energy and mass. But as the name suggests, it also taught  us that these properties are relative.

They appear to change as an object  moves relative to an observer. And one of those properties is mass. The faster you’re moving relative to someone else, the more massive they measure you to be.

Now for most speeds you run  into on a day-to-day basis, that increase in mass is inconsequential. It’s a rounding error. So for a lot of real-world  physics, and a lot of chemistry, you can rely on the simplified,  non-relativistic versions of equations that may have existed before Einstein came along.

But as the relative speed gets faster and faster, that increase in mass can’t be ignored. If you try to use the simplified equations,  your answers will come out wrong. Your predictions won’t match observations.

And that’s true whether you're a  futuristic spaceship trying to ferry humans across interstellar space  without them dying of old age, or you’re an electron in orbit of an atom. See, the more protons you  pack into an atom’s nucleus, the faster the electrons will be moving around it. That’s because protons are positively charged, while electrons are negatively charged.

Opposite charges attract, so with more protons there’s a stronger pull toward the nucleus. Now due to quantum shenanigans, like  the whole particle-wave duality thing, the electrons are still stuck occupying  those shells I talked about earlier. They don’t spiral down toward the nucleus.

They merely increase their velocity. So if we look at mercury, with its 80  protons, we see so much attraction that the two electrons in its innermost shell are  moving a little over half the speed of light! And half the speed of light is too fast  to ignore the rules of special relativity.

The electrons get a boost in mass. And that boost causes several  of the subshells to contract. The mercury atoms get smaller.

And remember, we’ve got a big  positively-charged nucleus tugging negatively-charged electrons. So if everything’s a bit closer together, the atom is going to hold onto  its electrons a bit more tightly. And that means it’ll be even more non-reactive than it was before you accounted for relativity.

Which mercury, of course, did  already have going for it. It’s a Group 12 transition metal  with completely filled shells. Just like its siblings zinc and cadmium.

But zinc only has 30 protons, and cadmium has 48. You can get away with ignoring  the effects of relativity when you’ve only got 48 protons. You can’t with 80.

Or at least that was the hypothesis  that started forming back in the 1970s. If scientists wanted to  prove that special relativity could explain mercury’s  especially low melting point, they’d need a computer that could handle  the simulations they’d need to run. Or more accurately, the two sets  of simulations they’d need to run.

The first set would create a  bunch of digital mercury atoms, and keep track of all the interactions  between them according to the simplified, non-relativistic  versions of our physics equations. Meanwhile, the second set would  instruct the atoms to obey the more complex relativistic equations. If the two simulations produced wildly different  melting points, with the relativistic one producing a temperature close to  what we observe in the real world, the researchers would know  they were onto something.

But that’s easier said than done. It’s hard enough getting a computer to  accurately simulate three interacting bodies, let alone a few dozen. But finally, in 2013, a team of  researchers were able to simulate a whopping 55 atoms of mercury.

When they ignored relativity, the  melting point was 82 degrees Celsius. That’s fairly low for a metal, but  nowhere near what it should be. But when they took the effects  of relativity into account, the melting point dropped to -23 degrees.

It isn’t quite the -39 we see in  the real world, but it’s way closer. In fact, it’s close enough to safely conclude that the puzzle behind mercury’s  sloshy nature had been solved. Still best if you don’t drink it though.

This episode of SciShow is brought to you by…you! Or some of you. Our Patreon supporters!

Thanks to them, we can keep the lights  on, the cameras rolling, the pre- and post-production teams typing and  clicking away on their keyboards. Really, this support is so very important. And if you want to be part of  this awesome group of people, head over to Patreon.com/SciShow. [♪ OUTRO]