[Music] hi it's Mr Anderson and this is AP Physics Essentials video 3 it's on the atomic nucleus um which was discovered by this man Ernest Rutherford JJ Thompson had already discovered the electron which they knew had a negative charge but they have viewed the atom as a plum pudding and this is what Plum Pudding looks like it's got little plums in it and then it's got the pudding portion so they they thought the atom looked like this it was the amorphous positive charge inside the atom and then these little negative charges were interspersed and so what rufer did is he shot alpha particles at it um those have a positive charge so we assumed they would just go right through since there was no positive centering of that positive charge that they would just kind of move through in a straight line but what he found is that these positive charges went straight on through but occasionally would come shooting right back at him or would be bent in some form and so um he said it was like shooting a shell at a Kleenex and that shell just came shooting back at him and so what he discovered was this really dense tightly packed positive nucleus uh which we know now is made up of positive protons but also these neutral neutrons now the protons are important because the number of protons tell us what the element is um in other words if it's got six protons we know that it's going to be carbon if it's got one proton then we know it's going to be hydrogen now you can have something that's the same element but varying amounts of neutrons and so if we add up the number of protons and neutrons and they're different in a given element we call those isotopes of that element and so for example carbon 12 is going to have six protons so is carbon 14 because they're both carbon but carbon 12 is going to have six additional neutrons and carbon 14 is going to have eight additional neutrons and so we're going to have these different isotopes and we'll find that for all of the different elements um some of these are radioactive and what that means is they're unstable and they have a potential to Decay or to fall apart they give off radiation when they do that and the rate at which they do that is known as their Half-Life and so again the atomic nucleus is made up of two subatomic particles we call those protons which have a positive charge and neutrons which have no charge you could count the number of positive charges right here and figure out what the element is um if we organize the protons in a certain fashion we get the periodic table and so we know that hydrogen has one proton that's what the atomic number means we know that iron has 26 we know that gold has 79 it tells us what the element is but you can have atoms of the same element and varying numbers of neutrons and when we do that we create something called an isotope and so if we look low on the periodic table we find uranium 92 that means it has 92 protons but there are going to be three naturally occurring Isotopes on our planet uranium 238 is going to have 146 neutrons uranium 235 will have 143 and uranium 234 will have 142 where did I come up with with those numbers if I take 238 which is the sum of neutrons and protons and I subtract 92 that tells me the number of neutrons and so on our planet we're going to have varying amounts of that and the average of that gives us the average atomic mass um now if we look at this graph right here this is graphing the number of protons along the x- axis and the number of neutrons along the y- AIS and so this would be that virtual straight line if the number of protons and the number of neutrons are equal and you'll find right here the average which is this jaggy line right here starts to drift towards the neutron side what does that mean as our atoms get larger and larger and larger and as our Atomic nucleus gets larger and larger and larger you have to have more neutrons to maintain the stability of the atomic nucleus and we'll talk about that in later videos but what you create and what the colors on this graph are are different types of Decay or Isotopes breaking down or they're giving off what's called radiation so radioactive decay is when the atomic nuclei breaks down and we could summarize that in three different types of radiation you have an alpha particle and that's two protons and two neutrons that are given off you could get a beta particle and that's either going to be an electron or a positron and then you have this high energy high level gamma radiation it's electromagnetic radiation so this would be an alpha particle given off by a by a nucleus and so it's become a different element so if we talk about an example of that uranium 238 naturally occurs on our planet um but it's going to undergo Decay it will lose a alpha particle and so as it loses that it's losing four of these nucleons these parts inside the nucleus and so you can see that the mass number has changed but it's also become a new element since you've lost two of these protons it's not uranium anymore it's thorium it could undergo then beta Decay so we lose a beta particle and it comes Pro actinium 234 it could lose another beta beta particle and it could become uranium 234 and so each of these have a probability of occurring and that probability is going to be in the atom itself and so if we take a sample of 238 we can create a curve of what it's what's called its Half-Life which is the ability for half of the atoms in that sample to Decay or to break down and so if we look at uranium 238 um at the beginning of time so time is graphed along Ong the x-axis at Time Zero we're going to have 100% of that uranium to 38 in one halflife that's what this one on the x-axis stands for we're going to have 50% of that uranium 238 Decay we're going to lose those alpha particles now what's the half-life of uranium 238 it's a ridiculously large number it's 4.5 billion years which is about the same age of our planet and so we would expect in that first 4.5 billion years we're going to have half of that uranium 23 create decay in the next 4.5 billion years we'll go from 50% to 25% and then to 122 and it'll keep following that at a known rate because there's a known probability of each of these atoms decaying during that period of time now what's cool about this is scientists can find a sample of uranium 238 we could figure out how much of it has decayed and we could find it along this line and that would tell us how long ago um that that uranium 238 let's say a rock for example actually formed and so did you learn to describe how the internal structure of a nucleus or an atomic nucleus um relates to the properties of that atom remember the protons tell us what it is and if we add the protons and neutrons we get the Isotopes which can be stable or unstable and can break down over time and I hope that was [Music] helpful a