The binding energy of ${ }_3^7\mathrm{Li}$ atom is $6.258\times {10}^{-12}\mathrm{J}$. If atomic masses of electron, proton and neutron are 0.0005 u, 1.0073 u and 1.0086 u, respectively, the actual mass of Li atom is

${ }_3^7\mathrm{Li}$ contains, 3p, 4n and 3e. Their sum of masses is

$m^{\mathrm{\prime }}=\left[3\right(1.0073\mathrm{u})+4(1.0086)+3(.0005\left)\right]\mathrm{u}=7.0578\mathrm{u}$

Mass corresponding to mass defect is

$\mathrm{\Delta }m=E/c^2=\left(6.258\times {10}^{-12}\mathrm{J}\right)/{\left(3\times {10}^8{\mathrm{ms}}^{-1}\right)}^2=6.953\times {10}^{-29}\mathrm{kg}$

In atomic unit, we have

$\mathrm{\Delta }{m}^{\mathrm{\prime }}=\frac{\mathrm{\Delta }m}{1.66\times {10}^{-27}\mathrm{kg}/\mathrm{u}}=\frac{6.953\times {10}^{-29}}{1.66\times {10}^{-27}}\mathrm{u}=0.0419\mathrm{u}$

Actual mass of Li is

$m=m^{\mathrm{\prime }}-\mathrm{\Delta }{m}^{\mathrm{\prime }}=(7.0578-0.0419\mathrm{u})=7.0159\mathrm{u}$