  Revealing the temporal evolution of individual heavy elements synthesized in
the merger ejecta from binary neutron star mergers not only improves our
understanding of the origin of heavy elements beyond iron but also clarifies
the energy sources of kilonovae. In this work, we present a comprehensive
analysis of the temporal evolution of the energy fraction of each nuclide based
on the $r$-process nucleosynthesis simulations. The heavy elements dominating
the kilonova emission within $\sim100$~days are identified, including
$^{127}$Sb, $^{128}$Sb, $^{129}$Sb, $^{130}$Sb, $^{129}$Te, $^{132}$I,
$^{222}$Rn, $^{223}$Ra, $^{224}$Ra, and $^{225}$Ac. It is found that the
late-time kilonova light curve ($t\gtrsim20$~days) is highly sensitive to the
presence of the heavy element $^{225}$Ac (with a half-life of 10.0~days). Our
analysis shows that the James Webb Space Telescope (JWST), with its high
sensitivity in the near-infrared band, is a powerful instrument for the
identification of these specific heavy elements.
