m_p = 1.0072765 amu
m_e = 0.00055 amu
m_n = 1.0086649 amu

n_p = 50
n_e = n_p
n_n = 124 - 50 = ?

m_{expected} = 123.9053 amu
n_p * m_p = ? amu
n_e * m_e = ? amu
n_n * m_n = ? amu
m_{calculated} = n_p * m_p + n_n * m_n + n_e * m_e = ? amu
n_p * m_p + m_n * n_n = ? amu
dm = m_{expected} - m_{calculated} = ? amu