hello everybody today I'm going to talk about average atomic mass I'm gonna give you an idea about how to calculate that other number on the periodic table now if you remember when we were talking the number at the top here is gonna be your lips sorry about that let's get the little pen up the number at the top of the periodic table that number right there is going to be your atomic number okay and as I said before you Tomic number is the number of protons obviously the symbol and then there's a number at the bottom with the decimal point and that is the average atomic mass average atomic mass okay and we're going to talk about how to calculate or actually in this video I'm going to talk about what average atomic mass is in the second video it shows you how to do the calculations okay so the number here again is not the mass number so don't get confused this number down here this decimal place number is not mass number not get this all the time students think that's the mass number I'm assuming that's coming from some previous class you might have had but it is not it's the average atomic mass so what that means is we're gonna take the average of something alright now this isn't the type of average that you've you might be thinking so what am I talking about okay it's get rid of the periodic table and let's start looking at what is average atomic mass okay first thing we need to talk about is the units for average atomic mass okay so if we talk about the atomic mass units okay because we are looking at dealing with individual atoms our atomic mass is going to be very small let's say we have one fluorine atom what's the mass of one fluorine let's just write the word one fluorine atom okay so if one atom of fluorine that mass is going to be three point one five five times ten to the minus twenty three grams now obviously you're going to get the idea that all masses for the grant or for atoms are going to be very very small to the negative twenty-something power so what scientists like to do is if we know that everything we're dealing with is going to be very very small we're gonna change our unit we don't want to use the unit of grams we want to change that to something different okay and the unit that we're gonna change that to is known as an atomic mass unit so atomic mass units and we're gonna breviary Tomic mass unit as a m u so this is a totally different unit well it's measuring the same thing but it's just a different measure right different measuring the scale I guess is what I'm looking for so we have pounds we have grams and now we have atomic mass units this is four and keep this in mind this is four atoms so this is for the size of atomic particles so if we're down to the size of looking at little tiny atoms then we're talking about AM use here's a little note we are only gonna use atomic mass units in this chapter I'll show you later on in the next chapter the next unit we cover how I go back from atomic mass units back to the mass okay and we're not gonna use this small number okay don't worry about it what you want to focus on is our brand new unit the atomic mass unit this atomic mass unit is based on carbon-12 I again you know I'm not really too concerned that you know though the origins of the unit I'm just concerned that you understand that it's a new unit okay so we've got the unit established so what am I talking about so let's say we have a piece of carbon okay a chunk of carbon so we got some carbon oops in that cabin but carbon solid okay so you got this piece of carbon solid you could hold in your hand it's a big chunk of it think of it in three dimensions big piece of solid carbon anyway so what is this carbon made up of okay it's gonna be made up of carbon atoms right okay it's gonna be made up of atoms of carbon but the problem is that we know that there are different isotopes of urban there's not just one type of carbon there's various isotopes so if I'm talking about this sample of carbon what I want to know is what is the mass of an individual carbon atom what's that mass gonna be okay so that's what I'm trying to figure out what's the mass and again I'm gonna do this and atomic whoops mass units okay that's my new unit now here's the problem since I don't have all of the same carbon atoms I have three different ones that have a different mass I have carbon-12 carbon-13 and I have a carbon-14 so any sample of carbon that you take is going to be broken up into three isotopes okay so one is gonna be a little bit lighter than the other two and that's gonna have a problem for our mass because we want to figure out well what's the mass of carbon that because if I have three different ones what do I do well since I have three different ones what I'm gonna do is I'm gonna take the average okay so I'm gonna take the average mass of all three now I know what you're thinking right away is you're gonna think oh I just add up all three and divide by three no you do not add them up and divide by three technically that's a shortcut to doing averages and the reason you can't do this is that these are not all evenly distributed I don't have an equal number of carbon-12 and carbon-13 and carbon-14 of the sample most of it is carbon-12 let's say that we have green be carbon 13 it's a little bit of carbon 13 and if I do blue carbon 14 very very small amount you can't even see it most of the sample is gonna be carbon-12 and if we say carbon-12 is the red one most of all of the carbon we deal with is going to be carbon-12 how do I know that because scientists have measured what's called the relative abundance ninety eight point eight two percent of our sample is carbon-12 I should take that back actually check the wrong number it would be eighty ninety eight point eighty nine not a big deal because significant figures that would be marginal anyway one point one percent of that sample is carbon 13 and a very very very very small amount of carbon-14 is there and it's going to be pretty much zero for this calculation based on the significant figures the percentage of this is going to be very low this is called the relative abundance II or relative amount sometimes they call it abundancy hopefully you know what abundancy means fundin si means amount so you can call it relative abundance E or a relative amount okay and it's always given as a percentage so what that means is that I have way more carbon-12 than I do carbon-13 and carbon-14 so if I were to try to figure out the average I can't just add them up because this contributes way more to the mass than the other two okay so what we have to do is we have to calculate the average so what I have to do is figure out the average by taking a weighted average so little note here this is a weighted average and to get a better color here this is weighted and what that means is that everything is not evenly distributed amongst all of the substances so in order to figure this out I need to have one more piece of information I need to know what the actual mass is because remember this is your mass number okay and mass number is not the actual mass right that's mass number it's just telling me that I have three different isotopes remember the only function of the mass number is to tell you the number of neutrons that's it it's all it tells us is the number of neutrons so we need the actual mass of carbon-12 well because the system for atomic mass units is based on carbon-12 this is gonna be my atomic mass darn it it's gonna be my atomic mass of carbon 12 is actually 12 completely perfectly right on the money because they decided that they made that the scale my carbon 13 is just slightly greater than the 13 and I know this is kind of gonna confuse unit before wait a minute isn't it so close that I could just say it's 13 yeah you yeah this is roughly what this number will be this is the only one that will be absolutely perfectly equal to that number so keep that in mind this is not exactly the true mass this is the real mess okay we don't need to worry about this one because carbon-14 is about zero so we're not going to worry about it in the calculation so what I need is the actual mass of the isotope the actual mass of isotopes and that's all the information I need to figure this out okay now one little quick thing you can explain here is this weighted great at the weighted average is what you do in my class my class is weighted grades 50% of your grade let's do it on another slide here if I'm looking at your grade in here your grade cannot you can't add up all the points in the class and divide it by the total number of assignments what you have to do is you have to realize that 40% say that again 50% of your grade is tests all right and you have 25% of your grade is quizzes and you have 25% for experiments so if you want to figure out your grade you have to figure out this and this and this and then multiply them by the percentages and then add them up that is exactly what we're going to do with average atomic mass and you'll see that calculation in the next video so just to recap here real quick what we have is we are looking at trying to figure out the mass of the atoms what mass are we going to use for atoms well in order to do that we have to look at the isotopes that make up our samples our carbons are magnesium's our aluminum's or whatever and each one of those will have a different number of isotopes and we need to know how much of each isotope we have which one do we have more of obviously because this one here is the most you know contributes the most when we look back at the periodic table and we actually look at that number at the bottom we can see that the mass number they actually the average atomic mass for carbon is gonna be very close to that number okay and you can see that's kind of the way it is for a lot of these closest to 27 closest to 28 because those are the most abundant isotopes you're gonna see some that kind of fall in the middle right here 106 point 4 it's kind of falling in the middle very close to 106 and you know 107 and 105 so hopefully that gives you a general idea of what average atomic mass is watch the other video that I have for the actual calculation and I'll see you guys soon thanks a lot