Why are large nuclei unstable?

Answer 1

So you wanna know why there aren't any stable atoms with atomic numbers greater than 83 (bismuth)? We're gonna find out, and to do so, we'll bounce around a bit in review and then fall on the answer. Buckle up. Ready? Let's do this. Atoms are comprised of protons, neutrons (in anything but "simple" hydrogen - 1H) and electrons. Set aside the electrons and let's look just at the nucleons. That's the name we give components of the nucleus, our protons and neutrons. Remember the basic laws of electrostatics? Like charges repel and opposites ones attract, right? Good. Let's jump. The atomic identity (sometimes called the proton number) of an atom is due solely to the number of protons in its nucleus. Only that. And in anything but 1H there are neutrons in the nucleus. Let's look at helium. It has two protons. Always. But it sometimes has a single neutron in its nucleus, and sometimes it has two neutrons. The one-neutron nucleus is very rare, and the two-neutron nucleus is super common. But look how it's made! You recall that when some hydrogen is squished down and turned into helium, that's fusion, right? Right. Now the news. Focus. The protons don't like each other. They're both positive, and repel. They'd rather not hang out together in a nucleus. But in helium, a neutron, or, most frequently, two neutrons, are "welded together" with the two protons to form the nucleus. What happens is that under extreme conditions (fusion), the protons and the neutron or two all go to Jenny Craig and give up some weight. This mass that they lose (called mass deficit), is converted into binding energy (or nuclear glue) to stick the whole thing together. That way we can get a stable nucleus with the two protons at least tolerating things. And the two different configurations, the one- and two-neutrons units, are called isotopes of helium. The word isotope speaks to atoms of a given element that have different numbers of neutrons in their nuclei. Got it? Good. Jump with me. In review, remember that whenever any heavier-than-hydrogen nucleus is formed by fusion, it must include neutrons. And all the nucleons that are going to be forming that nucleus (whichever one it might be) are going to 24-Hour Fitness. They're gonna be working off some weight (mass) to have it converted into binding energy. That's the only way to get the whole thing to stick together. Oh, and the binding energy thing is orchestrated by the strong force. That's new info. But don't get hung up there or you'll slide over to quantum chromodynamics (QCD), and it ain't time for that yet. As we build atoms bigger and better, it takes a few more neutrons at those higher atomic numbers to help create the binding energy. So when we get to heavier and heavier nuclei, the number of protons continues to climb, and the repulsive forces at work in the nucleus, the ones the binding energy is overcoming to keep the thing together, start to go outa sight. Eventually we simply can't make a heavier nucleus. The binding energy is insufficiently strong, even though we keep making more of it. And bismuth is the heaviest of the stable nuclei. Bummer. Oh, we can make heavier nuclei, we just can't keep them from just falling apart after a while. There are quite a few elements past bismuth. And they're all unstable, all radioactive with half lives of seconds to millions of years. All of them. All the elements and all of their isotopes. The mass deficit that creates binding energy will, at some point, be unable to overpower the repulsion of a large proton mass in the nucleus of a heavy element and keep it together. Nope, can't be done. Bismuth? Atomic number 89? End of the line for stable elements.

Answer 2

Large nuclei are unstable because the nuclear force decreases as protons spread farther apart.

Answer 3

because they stink