= Atomic physics
{wiki}
<Atomic physics> studies the structure and dynamics of <atoms>, particularly their <electrons> and their interactions with <radiation> and external fields. The <atomic nucleus> supplies charge and <mass>, but changes within the nucleus belong primarily to <nuclear physics>.
The <hydrogen atom> is a useful starting point because one <electron> moves in an approximately Coulombic potential. Its discrete <energy> differences produce <spectral lines>. More complex <atoms> add <electron> interactions, angular-momentum coupling and field-induced shifts. <Spectroscopy> tests those models by comparing predicted and observed <transition frequencies>.
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