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Nuclear Stability

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Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Nuclear stability is what makes certain isotopes radioactive. An isotope is unstable if it has a ratio of protons to neutrons that isn t within what is called the band of stability. Elements with atomic numbers greater than 70 are never stable. Unstable isotopes generally undergo transmutation, alpha decay or beta decay.

Alright so how do you know if something is going to be radioactive or not? How do you know of an atom or a particle is actually going to emit and be unstable enough to emit some alpha particles, gamma particles or beta particles. We're going to talk about nucleus stability and what makes something stable and what makes something a nuc- a nucleus unstable.

We have to talk about the there's there's a slight balance between electrostatic charges within the nucleus and a strong nuclear force within the nucleus we'll get to what that means in just a second, so we just think about it, we have a nucleus, inside the nucleus we have lot of protons and neutrons so let's talk about the protons.

All the protons are positively charged and we know that when two part two charges that come together that are like charges, when they come close they're going to repel each other so how is it that the the nucleus has all these particles of all these protons that are positively charged so packed closely together? Well that's because there's this something called the strong nuclear force and it doesn't have really a fancy name but this is what it's called but it's not really known a lot known about it quite yet but what this this force is, is that it acts on subatomic particles that are extremely close together and these this much stronger than our portion of these protons feeling from each other so this strong nuclear force is actually when they get really really close that repulsion is actually eliminated and they're actually going to be like [IB] together and if you think about it, there's a neutrons in there also, so the neutrons are have no charge so when they get close to protons that's okay but when so we like that connection that is [IB] strongly put together. When two protons come together we have they have to it's going to be negated the strong nuclear force is going to be a little bit less because they have to negate the fact that they're being pulled together they're being repelled I'm sorry because they have opposite charges.

So the nucleus, the neutrons also help keep the [IB] nucleus together so because we're talking about this balance between electrostatic forces that are keeping them apart and then and then the strong nuclear force is keeping them together we have to have a good ratio of neutrons to protons a stable ratio. If this ratio gets off balance that's when the nuc- the nucleus becomes unstable and they're also emitting particles so when this ratio [IB] in a good place. What does that mean ratio in a good place? So for low atomic numbers they found the that a really good ratio is a 1 to 1 ratio for every proton, you have a neutron and that is apparently very stable, but as you get in the larger larger atomic numbers you're actually going to need more neutrons to overcome those repulsions that protons they're feeling so we're going to have to need more so we're going to need 1.5 for every 1 one proton 1.5 nuclei sorry neutrons for every one proton so let's take a look at lead.

Lead is sometimes can mean different isotopes can be radioactive and different isotopes are not depending on the number of neutrons that are in it, so let's look at this one. This has mass of 206 and atomic number of 82. This atomic number tells me I have 82 protons and this mass number tells me that I have 124 neutrons 82+124 will equal 206 okay so let's find the ratio to see if this is an okay ratio. Well when I put a ratio of the neutrons on top of the protons it's 124 to 82 which then ends up to 1, 1.51 over 1 so the ratio was 1.5 this is a large atomic number that's okay this is actually stable one this is a maximum that it can be, if this was 208, then we start going under nuclear we have a nuclear reaction. Right this is 206 and the ratio is 1.5 so we're okay.

So how do I get those numbers as figyres? Well let's go over her. Here's what we call a band of stability here at the bottom line is the neutron to proton ratio is 1 to 1 so this is the slope of 1 this top line is what is the ratio of 1.5 to 1 so this is the slope of 1.5. Where this dotted line is in the middle this kind of fuzzy dotted line, that is where things are stable so if you find if you may figure out the neutron to proton ratio and you find that it's in this band of stability, then you're not going to have a nuclear reaction you're actually the nucleus is pretty stable okay but if you find that you're outside this like you're down here or you're up here, you're going to actually have a nuclear reaction or some sort of nuclear decay and it could be a fine thing it can happen even over millions of years and we're okay or it could be something disastrous so you want to make sure that we're in this band of stability to make sure that we have a stable nucleus and that's nuclear ability.