Nuclear mass defect
= Nuclear mass defect
{title2=$\Delta m=Zm_p+Nm_n-M_{\rm nuc}$}
The <mass> deficit of a bound <atomic nucleus> relative to its separated <protons> and <neutrons>. If their <rest masses> are $m_p$ and $m_n$, the <atomic nucleus> has <rest mass> $M_{\rm nuc}$, and the positive <nuclear binding energy> is $B$, then
$$
\Delta m=Zm_p+Nm_n-M_{\rm nuc}=\frac{B}{c^2},
$$
with $c$ the <speed of light>. Releasing <energy> when the <nucleons> bind reduces the <rest mass> of the bound system without changing the exact count $A=Z+N$. A neutral <atomic mass> also includes $Z$ <Electron> <rest masses>, reduced by the <energy> required to separate those bound <Electrons>. Consequently $A-m_{\rm a}/\mathrm u$ is not the definition of the <nuclear mass defect>: the reference unit and constituent <rest masses> must also be accounted for.