# I Reconciling alpha particle mass

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1. Dec 5, 2018 at 9:16 AM

### 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 at 10:00 AM

### 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 at 10:42 AM

### Orodruin

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

4. Dec 5, 2018 at 11:21 AM

### 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 at 11:36 AM

### Orodruin

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

6. Dec 5, 2018 at 1:03 PM

### BvU

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

7. Dec 5, 2018 at 4:01 PM

### mathman

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

8. Dec 5, 2018 at 6:30 PM

### PAllen

Helium 3 is stable and naturally occurring.

9. Dec 5, 2018 at 8:22 PM

### 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 at 8:31 PM

### 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 at 8:36 PM

### ezfzx

My point exactly.

12. Dec 5, 2018 at 8:37 PM

### ezfzx

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