  The subtle relation of the marginality of Dirac model and the emergence of
the Hopf term through a Yukawa-type interaction is revealed in this work. We
show that the improvement of the marginality of Dirac mode through an
infinitesimal non-relativistic term, which is irrelevant for renormalization
group, is necessary to emerge the Hopf term. It is found that the appearance of
the Hopf term is always accompanied with chiral boundary modes, and the sign of
the coupling constant of the infinitesimal non-relativistic term decides their
chirality. A lattice Dirac model is also constructed to realize the Hopf term.
