  A kilonova, the electromagnetic emission produced by compact binary mergers,
is formed through a delicate interplay of physical processes, involving
r-process nucleosynthesis and interactions between heavy elements and photons
through radiative transfer. This complexity makes it difficult to achieve a
comprehensive understanding of kilonova spectra. In this study, we aim to
enhance our understanding and establish connections between physical parameters
and observables through radiative-transfer simulations. Specifically, we
investigate how ejecta temperature and element mass influence the resulting
kilonova spectrum. For each species, the strength of its line features depends
on these parameters, leading to the formation of a distinct region in the
parameter space, dubbed the Resonance Island, where the line signature of that
species is notably evident in the kilonova spectrum. We explore its origin and
applications. Among explored r-process elements (31$\leq$Z$\leq$92), we find
that four species -- Sr$_{\rm II}$, Y$_{\rm II}$, Ba$_{\rm II}$, and Ce$_{\rm
II}$ -- exhibit large and strong resonance islands, suggesting their
significant contributions to kilonova spectra at specific wavelengths. In
addition, we discuss potential challenges and future perspectives in observable
heavy elements and their masses in the context of the resonance island.
