# 2.4 Atomic Weights

In addition to having varying number of electrons, atoms of a given element can also contain a varying number of neutrons, called isotopes. Since isotopes of an atom have different numbers of neutrons their mass is different.

2.4 Isotopes and Relative Atomic Mass Part 1

2.4 Isotopes and Relative Atomic Mass Part 2

### 10 Responses to 2.4 Atomic Weights

1. Alexandra Predmore says:

In the part of the second video about finding average atomic weight when you are subtracting 77.33 from 100 shouldn’t you get 24.67 instead of 24.47?

• fus says:

Hey Alex, you are correct in saying that the value should be 24.67. If you continue out the calculation, I did use 24.67 to get teh value of 35.45 amu, but wrote down 24.47 on the board. Sorry for the mix-up and thanks for pointing this out to me.

2. Janae Richardson says:

You only use the average atomic weight with isotopes correct? One of the problems on MasteringChemistry asked what is the average atomic mass of element X and gave four isotopes. I guess I was confused because I used the formula for average atomic weight and got it wrong. Can we do one of those in lecture?

• fus says:

Looks like isotopes and mass spectrometry is a topic we will definitely discuss in class tomorrow. See you there.

3. Christine Culbertson says:

When you refer the to abundance of an isotope, do you mean the abundance of an element on the entire planet? If so, how do we translate the composition of the Earth into something measurable on a mass spectrometer?

• fus says:

In nature, elements occur as a mixture of isotopes. What we report on the periodic table is the average atomic mass of the elements isotopes. In order to calculate this number we need to know a few things: How many isotopes exist, the mass of each isotope, and the relative abundance of each isotope.

I think what you are trying to get at here is that can we run every single, say chlorine atom on the earth through a mass spectrometer? We don’t but we can use statistics to report values to several decimal places. Remember that one mole of chlorine contains 6.022 x 1023 chlorine atoms. That’s a heck of a lot of atoms. So 6.022 x 1023 chlorine atoms weighs 35.453 grams. What if we took away just one chlorine atom? Would the mass change appreciably? How about if we were 10 chlorine atoms less than 6.022 x 1023? We will definitely discuss this tomorrow in class.

4. Jonathon Foreman says:

It should be noted that one cannot determine the isotopes directly from the periodic table. For some elements such as carbon, nitrogen, and oxygen the most abundant isotope is apparent. These are carbon-12, nitrogen-14 and oxygen-16, respectively. However, for other elements you’d be quite wrong to say the most abundant isotope has the mass close to the atomic mass. There is no bromine-80 for example. For all elements the actual abundance and mass numbers (and hence the number of neutrons) for isotopes must be attained from a source other than the periodic table.

5. Natika Washington says:

My brain is completely fried right now thanks to isotopes…where does these numbers come from as far as the abundance and atomic masses???? O.O???

6. Kaylee says:

How do I know how many isotopes an element has? Like is there a way to figure out Chlorine has two isotopes? or are we given this information… some of the mastering chemistry questions ask about this and I wasn’t sure if we were just supposed to look it up on the internet or know somehow? Thanks!

• fus says:

For this course you will either be given the number of isotopes or you will be given enough information to be able to determine how many isotopes there are of a particular element. Most of the time this is either listed in the problem or you can interpret a mass spectrum (pg 49 of the text).