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- [Instructor] In other[br]videos we have talked

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about that the type of[br]element that we are dealing

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with is defined by the number of protons

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in an atom's nucleus.

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So for example, any atom[br]with exactly one proton

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in its nucleus is by definition hydrogen.

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Any atom with six protons in its nucleus

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is by definition carbon,

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any atom with 17 protons in its nucleus

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is by definition chlorine,

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and so these numbers that I'm[br]circling on a periodic table

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of elements, that's known[br]as the atomic number,

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but it's really just the[br]number of protons in an atom

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of that element types nucleus.

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And that defines what[br]type of element it is.

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But in this video we're going[br]to dig a little bit deeper

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and realize that you can[br]still have different versions

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of the same element and these[br]versions in chemistry speak

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are known as isotopes.

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Now how can you have different[br]versions of the same element

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if the number of protons[br]defines what the element is?

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Well, the versions the various[br]isotopes are going to happen

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based on the number of neutrons you have.

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So for example, there are two[br]stable isotopes of chlorine,

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there's one version of[br]chlorine known as chlorine 35.

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Let me write it over here, chlorine 35.

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It's sometimes written like this,

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in fact it's often written[br]like this, chlorine 35

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and this isotope notation[br]that you see over here

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where we have 35 in the top left,

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that 35 is the sum of this version,

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this isotope of chlorines[br]protons and neutrons.

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This number 35 is this isotope[br]of chlorines mass number.

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So it has a total of 35[br]protons and neutrons,

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how many neutrons does this[br]version of chlorine have?

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Well it's going to have 17 protons.

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17 protons, I know that[br]because we are dealing

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with chlorine, so how many[br]neutrons will it have?

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Well 35 minus 17 is 18, 18 neutrons.

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And there's another version[br]of chlorine that is stable

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and that is chlorine 37.

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Now how many protons[br]is that going to have?

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Well that's a trick question,

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by definition it's chlorine,[br]it's going to have 17 protons.

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This is going to have 17 protons,

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but then how many neutrons will it have?

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Well the protons plus the neutrons is 37,

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so 17 plus 20 is going to be 37.

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So it's going to be 20 neutrons,

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and this would be written[br]out as chlorine, chlorine 37.

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So you can see these are two[br]different versions of chlorine,

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same number of protons[br]which make them chlorine,

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but different number of neutrons.

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Now you can imagine these[br]different versions are going

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to have different atomic masses,

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but here on a periodic table[br]of elements there's only one

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average atomic mass listed,

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and the key word here is this[br]is an average atomic mass.

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It's the weighted average of[br]the masses of the chlorines,

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the stable chlorines that you will find.

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So for example, in nature[br]75.77% of the chlorine

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found is chlorine 35,

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and then the remaining[br]24.23% of the chlorine found

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is chlorine 37.

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So when they calculate[br]this average atomic mass,

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what they do is they would[br]take, or you would take,

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if you're calculating it,[br]so this would be 75.77%

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times the atomic mass,

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atomic mass of chlorine 35 plus,

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and now the weight here would be 24.23%

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times the atomic mass,[br]atomic mass of chlorine 37.

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And if you were to do this[br]calculation you would get

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this number right over here, 35.45 unified

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atomic mass units.

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Now, how do you figure out the[br]atomic mass of chlorine 35?

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You might be tempted to[br]say it's just 35 unified

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atomic mass units, and you would be close

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because the mass of a[br]proton is close to one

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universal atomic mass unit,

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and the mass of a neutron is close to one

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universal atomic mass unit,

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and then the electrons[br]are have a much, much,

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much smaller mass.

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You can also almost[br]consider them negligible

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for atomic mass purposes,

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and so you will get an[br]atomic mass close to 35.

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But it actually turns out[br]it's a little bit different

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because not only are the masses[br]of each individual proton

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or neutron a little bit more[br]actually than one unified

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atomic mass unit,

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but when you put all those[br]protons and neutrons together

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in a nucleus, their[br]combined masses is actually

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a little bit less than[br]their individual masses

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if you were to just add them up,

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and that's actually[br]known as a mass defect.

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And so if you actually want[br]to know the atomic mass

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of chlorine 35, you can look[br]that up in a lot of tables,

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and you will see that it's[br]actually slightly under

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35 unified atomic mass units.