Main content
AP®︎/College Chemistry
Course: AP®︎/College Chemistry > Unit 1
Lesson 2: Mass spectrometry of elementsIsotopes
AP.Chem:
SPQ‑1 (EU)
, SPQ‑1.B (LO)
, SPQ‑1.B.2 (EK)
Isotopes are atoms of the same element with different numbers of neutrons. Because they contain different numbers of neutrons, isotopes have different atomic masses. The average atomic mass of an element is calculated by taking the weighted average mass of the element's naturally occurring isotopes. Created by Sal Khan.
Want to join the conversation?
At4:50-5:20
Any ideas as to why the mass of protons and neutrons together in the nucleus is a little less than one universal atomic mass unit? Even though individually, the mass of a proton of neutron is a little more than 1 universal atomic mass unit.(6 votes)
It’s because of mass defect/binding energy, basically some mass is lost as energy when the protons and neutrons come together to form the nucleus.(18 votes)
Why are protons not repelling in nucleus?(6 votes)
They do repel but are held together by the strong nuclear force which is stronger than the force of repulsion between the protons.(16 votes)
- How were the atomic numbers of elements determined?(2 votes)
The atomic number of an element is equal to the number of protons in a element
no. of protons = atomic no. of the element(5 votes)
Why aren't there any videos for the lesson on moles and molar mass? I know it's off topic, but I'm having trouble understanding moles & molar mass.(6 votes)
The mole is a unit for the amount of substance officially, or in other words how much of something there is. Similar to how a dozen of something tells you how many things there are, in a dozen's case 12. A mole is officially defined as 6.02214076 x 10^(23) particles, which is a very big number obviously. So if you have a dozen atoms you have 12 atoms, but if you have a mole of atoms you have 6.02214076 x 10^(23) atoms. Given how small and numerous atoms are we encounter them on the order of a mole often so it makes using it a convenient unit. Instead of saying I have 6.02214076 x 10^(23) atoms in this beaker, I can just say I have a mole of atoms in this beaker. This 6.02214076 x 10^(23) is also known as Avogadro's number, or Avogadro's constant.
Molar mass is the amount of mass contained within a mole amount of a certain substance. Molar mass is usually given in units of grams/mole, or g/mol. So if you have a molecule with a molar mass of 10.00 g/mol then what it's telling you is that if you gather together a moles amount of those molecules, 6.02214076 x 10^(23) molecules, then it will have a mass of 10.00 grams.
Hope that helps.(9 votes)
What determines as to whether an isotope is stable or not?(4 votes)- Atoms need a certain neutron-to-proton ratio to be stable. Essentially the protons and neutrons (collectively known as nucleons) exert a strong nuclear force on each other which counters to repulsive force of the positive protons in the nucleus. So if atoms increase in the number of protons with increasing atomic number, they need a certain number of neutrons to provide enough stabilizing strong force.
Stable isotopes have a neutron-to-proton ratio of ~1 for smaller atomic number elements and approach ~1.5. So stable isotopes have approximately the same number of neutrons and protons whereas unstable isotopes have larger differences in the numbers. Unstable isotopes are also referred to as radioactive isotopes and prefer to decay into more stable nuclei.
It should be noted that there is more nuance to this which includes nuclear binding energy. This is why chlorine's isotopes, Cl-35 and Cl-37, are stable but Cl-36 is unstable and radioactive despite all three having a similar neutron-to-proton ratio. So certain combinations of protons and neutrons are stable, while others are not.
Hope that helps.(7 votes)
I see he writes it like, "chlorine -35" except there's what looks like a line above I and n. What is that line for?(2 votes)
I think that was supposed to be him dotting the i.(4 votes)
- How did you get the percentages?(2 votes)
To get the abundance of each isotope, you could use a tool called a mass spectrometer. Basically how it works is that you have a stream of ionized atoms of one element. Then, you use a magnet to try and kick them out of their path. The ones that are the heaviest will be pushed less, and the lighter ones will be pushed more. You can measure where each atom ends up and use that to find out how often a particular isotope occurs. This khan academy video goes into more detail: https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:atomic-structure-and-properties/x2eef969c74e0d802:mass-spectrometry-of-elements/v/mass-spectrometry(4 votes)
- Does that mean that all elements are isotopes?(2 votes)
- All atoms of a certain element will be isotopes of that element. Eg consider carbon atoms on earth, some will have 6 neutrons, some will have 7, and some will have 8.(3 votes)
- How do we have any idea about the number of protons an element has?Or electrons even?(3 votes)
Number of protons is the atomic number of the element, and if it has not reacted then: Number of protons = Number of electrons.(1 vote)
So, the average atomic mass of say carbon is the mass of all carbon atoms in nature right?
And this mass is a weighted average based on the relative abundances of the isotopes of carbon.
The fact that the relative abundance (which are expressed as percentages) are used as fractions, makes the average mass not bigger than it's supposed to.
Am i right?(2 votes)- Yes, exactly right! Taking a normal average wouldn't make much sense because in it you assume that every isotope shows up an equal amount of time, which is wrong. We fix this by taking a weighted average of the masses, with the weight being the relative abundance of each isotope to get the average atomic mass.(3 votes)
4:50
Video transcript
- [Instructor] In other
videos we have talked about that the type of
element that we are dealing with is defined by the number of protons in an atom's nucleus. So for example, any atom
with exactly one proton in its nucleus is by definition hydrogen. Any atom with six protons in its nucleus is by definition carbon, any atom with 17 protons in its nucleus is by definition chlorine, and so these numbers that I'm
circling on a periodic table of elements, that's known
as the atomic number, but it's really just the
number of protons in an atom of that element types nucleus. And that defines what
type of element it is. But in this video we're going
to dig a little bit deeper and realize that you can
still have different versions of the same element and these
versions in chemistry speak are known as isotopes. Now how can you have different
versions of the same element if the number of protons
defines what the element is? Well, the versions the various
isotopes are going to happen based on the number of neutrons you have. So for example, there are two
stable isotopes of chlorine, there's one version of
chlorine known as chlorine 35. Let me write it over here, chlorine 35. It's sometimes written like this, in fact it's often written
like this, chlorine 35 and this isotope notation
that you see over here where we have 35 in the top left, that 35 is the sum of this version, this isotope of chlorines
protons and neutrons. This number 35 is this isotope
of chlorines mass number. So it has a total of 35
protons and neutrons, how many neutrons does this
version of chlorine have? Well it's going to have 17 protons. 17 protons, I know that
because we are dealing with chlorine, so how many
neutrons will it have? Well 35 minus 17 is 18, 18 neutrons. And there's another version
of chlorine that is stable and that is chlorine 37. Now how many protons
is that going to have? Well that's a trick question, by definition it's chlorine,
it's going to have 17 protons. This is going to have 17 protons, but then how many neutrons will it have? Well the protons plus the neutrons is 37, so 17 plus 20 is going to be 37. So it's going to be 20 neutrons, and this would be written
out as chlorine, chlorine 37. So you can see these are two
different versions of chlorine, same number of protons
which make them chlorine, but different number of neutrons. Now you can imagine these
different versions are going to have different atomic masses, but here on a periodic table
of elements there's only one average atomic mass listed, and the key word here is this
is an average atomic mass. It's the weighted average of
the masses of the chlorines, the stable chlorines that you will find. So for example, in nature
75.77% of the chlorine found is chlorine 35, and then the remaining
24.23% of the chlorine found is chlorine 37. So when they calculate
this average atomic mass, what they do is they would
take, or you would take, if you're calculating it,
so this would be 75.77% times the atomic mass, atomic mass of chlorine 35 plus, and now the weight here would be 24.23% times the atomic mass,
atomic mass of chlorine 37. And if you were to do this
calculation you would get this number right over here, 35.45 unified atomic mass units. Now, how do you figure out the
atomic mass of chlorine 35? You might be tempted to
say it's just 35 unified atomic mass units, and you would be close because the mass of a
proton is close to one universal atomic mass unit, and the mass of a neutron is close to one universal atomic mass unit, and then the electrons
are have a much, much, much smaller mass. You can also almost
consider them negligible for atomic mass purposes, and so you will get an
atomic mass close to 35. But it actually turns out
it's a little bit different because not only are the masses
of each individual proton or neutron a little bit more
actually than one unified atomic mass unit, but when you put all those
protons and neutrons together in a nucleus, their
combined masses is actually a little bit less than
their individual masses if you were to just add them up, and that's actually
known as a mass defect. And so if you actually want
to know the atomic mass of chlorine 35, you can look
that up in a lot of tables, and you will see that it's
actually slightly under 35 unified atomic mass units.