The actual measured mass of a potassium-39 atom is 38.963707 amu . What is the mass defect, Δm, for a potassium-39 nucleus?
The proton mass is 1.007276 amu , the neutron mass is 1.008665 amu , and the electron mass is 5.486×10−4 amu .
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Δm = { [Z * (m_p + m_e)] + [(A - Z) * m_n] } - m_atom
where
Δm = mass defect
m_p = mass of proton
m_n = mass of neutron
m_e = mass of electron
m_atom = mass of nuclide (A/Z) X
Z = atomic number
A = mass number
In this case, Z = 19, A = 39 and
Δm = { [19 * (1.007276 + 5.486×10−4)] + [(39 - 19) * 1.008665] } - 38.963707
= 0.3582604 amu