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Isotopes

# Isotopes

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?

• How were the atomic numbers of elements determined?
• The atomic number of an element is equal to the number of protons in a element
no. of protons = atomic no. of the element
• Why are protons not repelling in nucleus?
• They do repel but are held together by the strong nuclear force which is stronger than the force of repulsion between the protons.
• 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.
• 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.
• At -
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.
• 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.
• 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?
• I think that was supposed to be him dotting the i.
• Does that mean that all elements are isotopes?
• 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.
• How did you get the percentages?
• How do we have any idea about the number of protons an element has?Or electrons even?
• 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?