# The mass of a "_10^20Ne atom is is 19.99244 amu. How do you calculate its mass defect?

Jan 12, 2017

By taking the difference between the sum of the nucleon masses and the atomic mass.

#### Explanation:

I will not consider electronic mass here. We know (the masses should have been provided in the question!) that the neutron mass is $1.008701 \cdot \text{amu}$ and the proton mass is $1.007316 \cdot \text{amu}$.

Given $\text{10 protons}$ and $\text{10 neutrons}$, there combined mass is $10 \times \left\{1.008701 + 1.007316\right\} \cdot \text{amu} = 20.16017$ $\text{amu}$.

And thus the mass defect is, $20.16017$ $\text{amu} - 19.99244$ $\text{amu}$
$= 0.16773 \cdot \text{amu}$. And this seems a rather large value? I would check this with your text, or with someone who deals with particle physics problems. You are in the chemistry section, and certainly I don't have a feel for the right order of magnitude here.