Word problem

[jimmyp]

Europium is the element responsible for the red phosphors in a color TV screen. The mass spectrum of the isotopes of europium is shown below.

(a) How many protons are there in an atom of the most abundant isotope?

Here's a pictorial:
http://209.85.12.227/1318/19/upload/p10669585.jpg

[Sonic]

Isotopes do not affect the atomic number (number of protons) - if it has a different number of protons it's a different element.

Therefore it will be on the periodic table. Wikipedia says it's 63

[jimmyp]

I see. This means that it will also have 63 electrons in the MOST abundunt isotope, right?

How about when it comes to neutrons, how can I tell how many neutrons are there in an ATOM of the most abundant isotope?

[Sonic]

Most abundant/least abundant it will always be 63 electrons and 63 protons.

Neutrons are a different matter. Judging by the two peaks there exists only Eu-151 and Eu-153 with 47.8% and 52.2% relative abundance respectively.

The mean relative atomic mass is given by

[(relative abundance of Eu-153 x relative atomic mass of isotope Eu-153) + (relative abundance of Eu-151 x relative atomic mass of isotope Eu-151)]/100

You know:


relative abundance of Eu-153
relative atomic mass of isotope Eu-153
relative abundance of Eu-151
relative atomic mass of isotope Eu-151


Therefore you can work out the relative atomic mass. To find the number of neutrons subtract the atomic number (and round to the nearest integer depending on what your teacher says you should do)

[jimmyp]

Thank you very much. I see that technique now. One FINAL question on this problem:

If the mass of one isotope is 150.91985 amu and the mass of the other isotope is 152.92122 amu how do you calculate the atomic weight of europium and report it to the correct number of significant
figures based on the data given?

[Sonic]

You do the same thing I did in the code tags. Just change what you know. The number of significant figures should be the same as in the question, in this case 8

[jimmyp]

So I can write:

(0.522 * 152.92122) + (0.478 * 150.91985) = atomic weight of Europium

Did I do this calculation correctly? What does this number signify? And from which digit should I subtract it? (sorry about the repetitive questions, I just want to fully understand this question).

As for the number of neutrons can I just take the mass number of most abdundant isotope and subtract it from the atomic number of Europium:

152.92122 - 63 = Number of neutrons?