If you take a look at the periodic table, you’ll notice that lead is element number 82. Not coincidentally, every element with an atomic number greater than 82 is radioactive, meaning they’re unstable and prone to ejecting particles to seek stability. It has a lot to do with the number of neutrons and protons in the nucleus; to keep each other in check, their numbers should be roughly equal. If an element has too many neutrons, it’s at risk of shooting a particle away from itself to form a more stable substance.
Take a look on the flowchart at the right. We see here that Uranium, at the top of the list, has an atomic number of 92, meaning it has 92 protons. The number “238” refers to the sum of uranium’s protons and neutrons. With some quick subtraction, we can deduce uranium has (238 – 92) = 146 neutrons. Wowzers! It has about one and a half times as many neutrons as protons, so what could we say about the stability of uranium? Well, if you’ve any inkling for radioactivity, you probably already know that uranium tends to decay and is therefore highly radioactive. It’s the mystique of such heavy atoms that has placed these radioactive species at the forefront of science fiction, appearing in all sorts of fantastic gadgets like time machines (what was the element they needed in Back to the Future? Ah, yes, it’s plutonium, atomic number 94, just two doors down from uranium. No surprise there!)
You might be asking yourself, “so what kind of particle might uranium want to eject to become stable?” The answer is that it’s complicated. Radioactive elements generally have 5 ways of ejecting particles — sometimes they shoot off an electron, sometimes they shoot off a proton, or they might try their luck at adjusting their neutron number. You can see on this flowchart that uranium ejects an alpha particle, which is nothing more than a helium nucleus (helium has 2 protons and 2 neutrons, and since this is the nucleus, there are no electrons emitted). If uranium shoots out two protons, it loses its identity as it drops its proton count down to 90. Now it’s thorium! Notice the huge amount of time listed there: that’s the half-life of uranium decay, meaning that in 4,510,000,000 years from now, a one pound sample of uranium will still be half uranium by weight. It takes quite a long time, but check out what’s next in the line-up.
Thorium ends up emitting an electron from its nucleus. Wait, what? There aren’t any electrons in the nucleus, so how can this occur? It might blow your mind: one of its neutrons splits into a proton and an electron. The electron is emitted while the proton remains behind. Since we’ve added one more proton, our element becomes protactinium, and it happens in just a few weeks!
This series of events will continue for quite some time until it reaches the end of the line, which you’ll notice is Pb, or lead. I don’t think it’s a coincidence that lead is the last non-radioactive element in the periodic table — by definition, radioactive elements will always spit out some particles until they become less and less unbalanced. It just so happens that uranium will always, always, always eventually decay into lead.
So in a few trillion years from now when we’re mining the far reaches of outer space and living in Jetsons-esque societies on a hundred different planets, nearly all the uranium that exists today will be on its way to become lead. If we find some elements that are even more radioactive than the ones we’ve already discovered, at least we have some consolation: all that lead converted from uranium will do nicely as a shield from x-rays and other radioactive energy. Kind of ironic, huh?
Your Brain on Sci 
