Average atomic mass (video) | Khan Academy

Video transcript

– [ Instructor ] The thing that I ‘ve constantly found amazing about chemistry, it ‘s an entire plain of science that we as human beings have developed to actually understand what is happening in an about unimaginably belittled scale. In especial we ‘re gon na be thinking about the nuclear, and even the subatomic scale. And by looking at that scale we can then begin to understand the population in which we live in, the scale in which we live in, and even be able to make predictions about what will happen, and make things that are utilitarian for human beings. so if we ‘re going to operate at this little of a scale, and we ‘re gon na appreciate in a few seconds how small of a scale it is, we ‘re going to have to have some units of measurement. And this video is going to focus on mass. How do we measure mass at such a humble plate ? Well to do that the chemistry community has historically used something called an atomic mass unit. I ‘ll write it here, nuclear, atomic mass whole, and it ‘s historically denoted as AMU. And more recently, the more advanced translation of this is the mix atomic mass unit, that is denoted by just a uranium alternatively of an AMU. So how does a unite atomic mass unit connect to our units of bulk that we might use on a larger scale like, say, grams or kilograms. Well, the coordinated atomic mass unit is defined as 1.660540 times 10 to the negative 27 kilograms. thus when you see something like this, you might have a few reactions. Your first reaction, which would be an appropriate reaction, is that belly laugh, 10 to the negative 27 power is very little. To appreciate it you could write it out, it would be zero point and then 26 zeros and then you would have one six six zero five four zero. then very, very, very belittled, very unimaginably small. We can merely try to abstract it with things like mathematics. The other thing you might appreciate is this feels like a morsel of a hairy phone number here, 1.660540, why did they define it that way ? And the answer to your wonder is, this definition makes it a distribute cleaner when we think about the mass of whether it ‘s an atom or the constituents of an atom like a proton or a neutron. approximately speaking the mass of a proton is approximately one unite atomic multitude unit. The multitude of a neutron is approximately one unite nuclear mass unit. It actually turns out that a proton ‘s a little bite more than one, it ‘s about 1.007 nuclear mass units, but it ‘s approximately one. And the neutron is actually a little bit more than even a proton, it ‘s 1.008 approximately unite nuclear batch units. now an electron ‘s aggregate is actually far smaller than either of these, it ‘s actually about one two thousandth of a proton or a neutron, and thus you can imagine an atom which is made up of protons and normally neutrons and electrons vitamin a well, the mass is chiefly going to be the protons and neutrons in the nucleus. And so if you know the phone number of protons and neutrons in the nucleus, you ‘re going to have a reasonably commodity sense of its nuclear batch. And you can see that indicated on a periodic table of elements which we have here. And we will study the periodic table of elements in a set more detail in early videos. But you can see a copulate of interesting elements. One, you have the abbreviation of a given element, H represents hydrogen. The number on top on this periodic table, that ‘s the nuclear total, and that tells you how many protons it has. And an element is defined by the number of protons. so any atom that has precisely one proton in its nucleus is going to be hydrogen by definition. Any atom that has precisely 20 protons in its nucleus is going to be calcium by definition. Any atom that has precisely 36 protons in its lens nucleus is going to be krypton by definition. So what would you expect the mass of a hydrogen atom to be ? Pause this video and think about it. Well we know that all hydrogen atoms by definition have one proton, but it actually turns out there ‘s different versions of hydrogen that can have different numbers of neutrons. Most of the hydrogen in the universe actually has zero neutrons, zero neutrons. There are versions that have one or two neutrons, but most, 99.98 % roughly, of hydrogen in the universe has one proton, zero neutrons, and if it ‘s a inert hydrogen it ‘s going to have one electron. And when we talk about versions of a given chemical element there ‘s a fondness word for it, they ‘re called isotopes. And the different isotopes, they ‘ll all have the lapp number of protons ’cause they ‘re talking about the same component, but they ‘ll have different numbers of neutrons. And so if this is the most common form of hydrogen. What do you think its bulk is going to be ? Well its multitude is going to be basically the mass of a proton plus an electron, and roughly speaking it ‘s going to be the multitude of a proton ’cause the bulk of a proton ‘s going to be sol much larger than the mass of an electron. And so you would expect that its batch is approximately one mix nuclear multitude unit. now if you were to precisely look at the mass of a proton and a electron, if you add them together, you actually get something that ‘s a small bit closer to 1.008. And you actually see that correct over here on the periodic postpone of elements. now this count, although it is reasonably conclude to the bulk of the translation of hydrogen that I just described, it ‘s actually a slant average of the diverse versions of hydrogen. It ‘s just close to this version because this translation represents most of the hydrogen that we actually see around us. If for example you had two versions of an element, some conjectural element, and let ‘s say that 80 % of the component that we see is version one and version matchless has a mass of lashkar-e-taiba ‘s call it five nuclear mass units, and then version two, it ‘s the remainder, 20 %, of what we observe of that chemical element, it has an nuclear mass of six atomic mass units. You would get a slant average here of 5.2 unite atomic mass units. And that ‘s actually how these numbers are calculated. They are not just the bulk of one type of that element, they ‘re a burden average mass of the respective isotopes, of the respective types. And so this number on a periodic board of elements is known as the average nuclear bulk, average, average atomic atomic mass. now in older chemistry books, and this is actually the case when I inaugural learned chemistry, they call this act atomic burden. And I ‘ve always complained about it because it ‘s truly talking about mass and not weight. If you do n’t know the remainder you ‘ll learn that at some point in the future, and it ‘s actually talking about average nuclear mass. now I ‘ll give you one small detail that might be useful to you. sometimes you ‘ll hear something called proportional nuclear batch. It actually turns out this periodic table of elements, because it does not write a little U after each of these numbers, it ‘s basically these number are unitless, so it ‘s in truth talking about relative nuclear batch. So it ‘s saying, hey on median, for model, the bulk of a carbon atom is going to be roughly 12 times that of, on average, the mass of a hydrogen atom. If they put the units here, then that would actually in truth be average atomic mass. But for our purposes, as we go into chemistry, you can look at these numbers, and say okay, if oxygen has a proportional atomic mass of 16, it ‘s average atomic multitude is going to be 16 unite nuclear mass units. And as we will see in the future, this understanding of average nuclear mass will prove to be very, very useful.

source : https://thaitrungkien.com
Category : Tutorial

Related Posts

Trả lời

Email của bạn sẽ không được hiển thị công khai.