= Solution
Away from an absorption edge, the scalar <atomic scattering factor> is the <Fourier transform> of an approximately spherical, real atomic <electron density>. Opposite points contribute conjugate phases, so their sum is real. In <Thomson scattering> the electronic response is also effectively in phase with the driving field in this nonabsorbing approximation. Near an absorption edge, resonantly driven electronic excitations introduce a phase lag and absorption. The <atomic scattering factor> becomes $f_0+f'+if''$, with $f''$ describing the absorptive component and $f'$ its dispersive companion. The sign of the imaginary part depends on the time and scattering-phase conventions; here it is the source's $+if''$ convention. This discussion concerns scalar charge scattering, without magnetic or tensor resonant contributions.
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