Alpha decay | Physics | Khan Academy

why doesn't our periodic table go on forever why don't we have for example elements with 300 protons so say a th000 protons well the short answer is because heavier the elements the more unstable they become for example elements about um atomic number 83 they don't even have a stable isotope all their Isotopes are radioisotopes which means sooner or later these will Decay and if you consider even heavier elements well they will Decay almost instantly and there's no chance that they exist in nature but why are they unstable in the first place and how do they

undergo dek and how do we use this to create smoke detectors let's find out so let's start by asking ourselves why do heavy nuclei become unstable in the first place well the answer is because there are two forces of nature act play over here which are acting against each other the first one is the good old kums repulsion the electric force all the positive protons are repelling against each other this is what tries to blow the nucleus apart but then what keeps the nucleus together in the first place Well turns out there is a force

even stronger than that the strong nuclear force and this force is an attractive force and that's what keeps the nucleus together and it does so because strong nuclear force has two advantages over the electric force the first one is it is the strongest force of nature so that's amazing but there's another one you see the kum's repulsion the electric repulsion is only between the protons because it's only between charged particles neutrons do not participate right however when it comes to strong nuclear force both protons and neutrons participate so they're all involved in the nuclear strong

nuclear attraction this is why a lot of stable nuclei exist in our nature because the strong nuclear attraction just overpowers the electric repulsion or the kums repulsion but compared to the electric force the strong nuclear attraction has one disadvantage that is it is a short ranged Force what does that mean well if you concentrate on just say one proton then it is being repelled by all the other protons in this nucleus regardless of how far the other protons are because electric forces are long ranged it doesn't matter how far they are the electric force works

and so it's being repelled by all the other protons but the nuclear force is short ranged this means even though it is super strong and everything um only the few protons and neutrons in its vcinity they are the ones in its neighborhood they're the only ones that can attract it the ones that are far away well they can't reach it their nuclear forces are short ranged now for lighter nuclei this is not a problem because it is very small so most of the protons and neutrons are within the nuclear range but what happens as the

nucleus gets heavier well just imagine an extreme case if there were lots and lots of protons and neutrons all the protons will repel it so the electric force becomes incredibly large on this proton but not all the particles are going to attract it only the ones in the neighborhood are going to attract it which means if you have too many particles and the nucleus is too big eventually electric force will overpower our strong nuclear force and that's what will make it unstable and so now hopefully you can see how or why as the nucleus becomes

more and more heavier it becomes bigger and bigger and because of the short range nature of the nuclear force eventually electric force can win out that's what makes these things unstable so if you now imag imagine elements with thousands of protons there's no chance that they will stay put the electric force will just blow it apart it'll just break instantly but what about the heavy elements that we do have in the periodic table well over here the strong nuclear force can sort of kind of hold on to them but not forever they have found a

way to become more stable how well they just spit out a helium nucleus and we call this the alpha DK we call it the alpha DK because when we first discovered it we didn't know which this particle was what particle this was and we just called it the alpha particle but later on we realized that it's just a helium nucleus with two protons and two neutrons okay this might raise a lot of questions now the first question could be how does this make thing make things stable well you can see the daughter nucleus now has

less particles in it so it's slightly smaller than the parent nucleus if it's smaller it's that much easier for the strong nuclear force to hold on to it and therefore it becomes more stable than the parent nucleus now that is doesn't mean that this is completely stable this might still be a radioisotope and it might further undergo more Alpha DEC which is totally possible okay but then a followup question that comes to my mind is why a helium nucleus why not anything else why does it spit out precisely this well the short answer is because

helium nucleus is incredibly incredibly stable and so it's just more energetically favored and therefore that's what happens okay another question we could have is When Things become stable the energy decreases right this should now have less energy compared to the parent nucleus that's what it means to be stable but if that's the case where did the energy go well that energy comes out as the kinetic energy of these particles the alpha particle will take up most of the kinetic energy but the daugh nucleus will also have some recoil as well and now guess what these

alpha particles can go and hit other atoms make them jiggle causing Heat this is how radioactive heating works and fun fact folks we believe this is majorly what keeps the Earth's core prettyy hot I mean there are some other reasons but we think this is the major one and another fun fact this is where most of the helium on our planet comes from from the radioactive DK of elements found inside the earth anyways let's now familiarize ourselves with this and take a couple of examples one example could be uranium 92 turns out it will undergo

an alpha decay the question for us is what happens after the alpha dekay can we predict what the daughter isotope would look like well let's see here's how I like to think about it I know if it's an alpha DK then a helium nucleus com comes out therefore my daughter nucleus must have two less protons in it it started with 92 protons two less protons which means that daughter nucleus should have 90 protons in it similarly uh my daughter nucleus will now have four particles Less in total it started with 238 now it has four

particles less that means it should now have 234 particles in it that should be its new mass number so my new my daughter nucleus should look like this but what is it I don't remember which what is the element it's not uranium anymore because it is 90 and I don't remember you don't have to remember that's where we have that's why we have the periodic table if I just look at the periodic table I see 902b over here so this is Thorium thorium so look we get a new element Al together and if you look

at the periodic table you can see we've gone from uranium to thorium so that means we hop two to the left in the periodic table that kind of makes sense because you're losing two protons so you hop two elements to the left okay why don't you try one in this example let's say there is some parent nucleus that undergoes an alpha DK and gives you neptunium 93237 can you pause and predict what the parent nucleus be it's it's you know it's atomic number and the mass number when you pause and try all right again it's

an alpha DK so I know a helium nucleus must have been thrown out therefore I can now predict well there is 93 in the daughter two came out so the parent had to have 93 + 2 95 protons in it and similarly four came four particles came out 237 is in the daughter nucleus that means the parent must have had 237 + 4 241 total number of particles in it and therefore my my parent nucleus would be having an atomic number 95 which is over here am am stands for am and so there you have

it that's my parent nucleus and again you can see if you look at the periodic table after the alpha DEC we've gone two elements to the left of the periodic table so you see all we have to do is keep track of total number of protons and neutrons they stay the same because nothing is changing the total number of protons and neutrons here will be the same as the total number of protons and neutrons here and if you keep track of that we'll be able to predict what our daughter nucleus would be or what our

Paran nucleus would be now there is a reason why I took this example because this is the radio isotope used in smoke detectors but how does alpha DK help us detect smoke let's see the heart of these radioactive smoke detectors will contain two plates connected to a battery so that they have a positive and a negative charge and you have the amarium source at the bottom now the amarium is going to give out a lot of alpha particles but so far did you notice something about the alpha particles they have two protons in them but

they don't have any electrons because they came from the nucleus which means alpha particles have a plus two charge on them and since they're moving with a high speed they can now knock off a lot of electrons from the atmospheric particles over here from the oxygen nitrogen and all the other atoms and that's why we'll now end up with a c of electrons and positive ions now of course the helium nuclei will eventually pick up electrons and become neutral atoms but eventually they leave behind a lot of positive and negative charges as a result this

whole thing becomes a conductor now and therefore there will be a current a continuous current running in the circuit now this process where the helium nucleus is able to knock off electrons is what we call ionization and that's why we call Helium to be highly ionizing radiation but anyways what happens when we have smoke well it disrupts this process it doesn't allow the helium to ionize a lot of these atoms anymore because of the smoke particles in between that significantly reduces the current and that is sensed by a sensor in the circuit and the alarm

goes off this is how alpha DK can be used to detect smoke I find this absolutely mindboggling but you might be concerned thinking that we have a radioactive element in our house now isn't that dangerous well in general radioactive elements are dangerous but here's the thing about alpha particles sure they are extremely ionizing however they don't go very far they can be easily stopped Say by just a piece of paper because they're so bulky and therefore although they have a high ionization power they have very low penetration power and that's why none of the alpha

particles will even leave the casing of your smoke detector and so you don't have to worry about anything but what if you decide to Now open up the smoke detector to have a closer look at the amarium well now that can be dangerous because you might have some neptunium 237 lying around somewhere over here floating around you might ingest it neptunium is also radioactive and therefore now you have radioactive elements inside your body that can be dangerous so don't do that