# I Reconciling alpha particle mass

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1. Dec 5, 2018

### ezfzx

Ugh ... I remember there was a reason ... but forgot what it was.
So here's 3 bits of information gathered from dozens of sources, textbooks, official sites:
1. Alpha particles are identical to Helium nuclei.
2. Alpha particle mass = 4.001506 u
3. helium-4 nuclei mass = 4.0026032 u
OK, so ... why aren't they the same?

2. Dec 5, 2018

### BvU

You sure about bit 3 ? Isn't that the isotope mass instead of the nucleus mass ? So 2 electrons minus some energy heavier ?

3. Dec 5, 2018

### Orodruin

Staff Emeritus
You are referring to the atomic mass, but yes, it is.

4. Dec 5, 2018

### Staff: Mentor

To make a more explicit hint: find the difference between the two masses, convert to kg, and divide by 2. Does the resulting number look familiar?

5. Dec 5, 2018

### Orodruin

Staff Emeritus
I will convert to keV/c2 if you don't mind ...

6. Dec 5, 2018

### BvU

Considered it a reasonable term, since the atomic mass is normally an average over isotopes.

7. Dec 5, 2018

### mathman

He has only one stable isotope and 3) is the atomic mass, not nuclear.

8. Dec 5, 2018

### PAllen

Helium 3 is stable and naturally occurring.

9. Dec 5, 2018

### ezfzx

OK, so the extra mass is the electrons.
That's fine. But that extra mass gets tacked on when whenever I go searching for the mass of the He-4 "nucleus".
Every source seems to ignore the fact that I'm asking for the "nucleus".
If I'm building nuclear "binding energy" questions, I need reliable numbers.
If the source isn't specific about "atomic" vs. "nuclear" mass ... or worse ... if the source is listed in response to my specific search for "nuclear mass", it can be a little frustrating. I can compare He-4 to an alpha, but what about all the other elements?

I guess the point of the story is to get values from a reliable source.

10. Dec 5, 2018

### Staff: Mentor

You also found the mass of the nucleus.
Just make sure you check which value is given in a source. A proper source will tell you that.

11. Dec 5, 2018

### ezfzx

My point exactly.

12. Dec 5, 2018

### ezfzx

Kids ... don't get your data from the back of a truck in a dark alley!