Relativity & chemistry

Apologies in advance for this overly verbose post, but it shows how something as mundane as starting your car hinges upon relativity. I hope this post doesn’t put you to sleep, or worse, make your eyes bleed, but I think it’s worth the journey to understand a little more about the physical universe in which we live.

Relativity is counterintuitive to us because we never directly, knowingly experience relativistic effects, and yet those effects are all around us.

Take a 12V lead battery as an example. Roughly 10 of those 12 volts we take for granted occur because of relativistic effects, which is surprising. In other words, relativity is much more commonplace than we first thought, and it’s worth understanding why.

First a little high school chemistry recap… Elements in the periodic table are listed in order of their atomic number (which is the number of protons in the nucleus of an atom). Elements are grouped or stacked “periodically” meaning according to shared/similar chemical properties.

Look at tin (Sn) and lead (Pb), they’re in the 14th column, rows 5-6.

Tin (Sn) has 50 protons so it’s atomic number is 50, while Lead sits directly below Tin, having 82 protons.

The columns in the periodic table represent the number of electrons in the outer shell of each particular element. Although all these elements have a different total number of electrons, those in the same column have the same number of outer electrons, and so have similar chemical properties.

periodic table

Why is this important? Well, when it comes to our lead battery, in theory we should be able to make batteries out of tin as well as lead. After all, they’re chemically very similar. The problem is… in practice we can’t. And this inconsistency got scientists thinking, wondering why there would be such a stark difference between two elements that are chemically so similar.

The answer is… relativity.

Opposites attract, so electrons (-ve) “orbit” protons (+ve). Actually, electrons don’t swing around the protons at the heart of an atom like a planet orbiting the sun, they frequent a probabilistic cloud, but for our purposes we’ll talk of them orbiting.

Now, the larger atomic number, the more +ve protons there are in the atom’s nucleus. Remember, tin has 50 protons, lead has 82, so lead has considerably more positive charge attracting negative electrons. And here’s where relativity kicks in. With so much charge, the negative electrons whizzing around such a large positive charge travel at 60% of the speed of light, and as Einstein showed with E=mc2, that blistering speed means these more energetic electrons are more massive that those orbiting the nucleus of a tin atom!!!


Think about water rushing down a hole. The larger the hole, the more water can flow through. Water falls into the newly-formed well around the hole. In the same way, protons form an energy well. The more positive protons there are at the heart of an atom, the faster they draw in electrons, only unlike water, the electrons never reach the protons, they get trapped swirling around them in a cloud.

And this is where relativity comes into play, our electrons are moving so fast their mass increases and they drop deep within this energy well. So when they’re released in a chemical reaction, the electricity that’s liberated is significantly more for lead than for tin (deeper well = faster electrons = more energy).

In science fiction, we’re used to hearing about the speed of light as a limit on spaceships traveling between stars, but the speed of light is intricately tied into the very fabric reality, having a significant impact on chemistry.

The chemical properties of heavy elements are not just determined by their charge but by the speed with which electrons orbit their nucleus, and that’s a surprising insight.

Are you married? Do you have your wedding ring on? The gold in your wedding ring has that beautiful shine because the electrons whizzing around the nucleus are racing along at over half the speed of light! As with lead, the insanely fast outer electron hurling “around” the gold nucleus causes this element to behave differently to those lower on the periodic table.

With one electron in its outer shell, we’d expect gold to be highly reactive, like other elements that have only one outer electron. Lithium, sodium, potassium, copper, silver, etc are all highly reactive, tarnishing in the air and in some cases exploding on contact with water, and yet gold doesn’t share this behaviour even though strictly it should. Gold is renown for being inert. Why? Relativity.

That one outer electron is buzzing around so fast its mass has increased by 20% causing it to fall inward and almost join the next inner shell of electrons, making gold impervious to chemical reactions.

All this brings up an interesting point, if the speed of light were lower, chemistry would change. If the speed of light were half what it is, elements like uranium could not exist, gold and lead would be highly radioactive and unstable like uranium and plutonium, while elements like potassium or copper would take on the properties of gold.

So next time you put on your wedding ring or start your car, remember you’re interacting with elements that only exist as they do because of relativity.

Pretty trippy, huh?


9 thoughts on “Relativity & chemistry

  1. 0_0 Whaaaaaat? You just trip up a lot on mind. Actually you make it worse. My thoughts got a blue error screen from old Windows. I knew particles behaved like large objects in space and all, but that perspective was a lot.

    In the chemistry topic on batteries: is olivine really better than lithium?

  2. Very nicely explained. 🙂

    My endlessly speculative and skeptic mind has me wonder, though… If the speed of light were half of what it is, would electrons really lose their speed, or simply be faster than light? Is there a dependency there, or simply a comparison between speeds (since light is our upper limit so far, and thus our term of comparison)?

    Great post, Peter, and yes, it’s absolutely fascinating how much the speed of a couple of electrons can change the way substances behave and affect our lives. 🙂

    • Yeah, it’s pretty wild to think that 80% of the energy we liberate from a lead battery comes from this miniature “energy well.” It’s hard to imagine that something you can hold in your hands has electrons orbiting at such breakneck speeds that their mass substantially increases.

      If the speed of light were lower, assuming E=MC2 still holds in our hypothetical pocket universe, then there would be a lower speed limit, but it would be just as uncross-able as so long as something has mass it can’t reach the speed of light. So electrons would get closer to the limit and thus be more massive but would never go beyond that limit. I suspect they’d pack more of a punch when liberated.

    • I find it surprising/astonishing to think that if the speed of light were double what it is, the universe would be radically different. And it’s not just that gold would tarnish and lead batteries would be considerably weaker, as these are just the obvious examples we’ve explored so far. From what I’ve read, I suspect the relativistic effects on chemistry go deeper, as any two elements in the same column on the periodic table should be chemically identical except for their weight, and yet they’re not. I wonder if we’ll learn of about subtle relativistic effects distinguishing the elements as we study this more. Fascinating.

  3. Pingback: The Periodic Table | Cool Story Bromine

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