  Using the tight-binding method in combination with first-principles
calculations, we systematically derive a low-energy effective Hilbert subspace
and Hamiltonian with spin-orbit coupling for two-dimensional hydrogenated and
halogenated group-V monolayers. These materials are proposed to be giant-gap
quantum spin Hall insulators with record huge bulk band gaps opened by the
spin-orbit coupling at the Dirac points, e.g., from 0.74 to 1.08 eV in
Bi\textit{X} (\textit{X} = H, F, Cl, and Br) monolayers. We find that the
low-energy Hilbert subspace mainly consists of $p_{x}$ and $p_{y}$ orbitals
from the group-V elements, and the giant first-order effective intrinsic
spin-orbit coupling is from the on-site spin-orbit interaction. These features
are quite distinct from those of group-IV monolayers such as graphene and
silicene. There, the relevant orbital is $p_z$ and the effective intrinsic
spin-orbit coupling is from the next-nearest-neighbor spin-orbit interaction
processes. These systems represent the first real 2D honeycomb lattice
materials in which the low-energy physics is associated with $p_{x}$ and
$p_{y}$ orbitals. A spinful lattice Hamiltonian with an on-site spin-orbit
coupling term is also derived, which could facilitate further investigations of
these intriguing topological materials.
