  Thermalization of isolated quantum systems is a long-standing fundamental
problem where different mechanisms are proposed over time. We contribute to
this discussion by classifying the diverse quench dynamical behaviours of
spin-1 Bose-Einstein condensates, which includes well-defined quantum collapse
and revivals, thermalization, and certain special cases. These special cases
are either nonthermal equilibration with no revival but a collapse even though
the system has finite degrees of freedom or no equilibration with no collapse
and revival. Given that some integrable systems are already shown to
demonstrate the weak form of eigenstate thermalization hypothesis (ETH), we
determine the regions where ETH holds and fails in this integrable isolated
quantum system. The reason behind both thermalizing and nonthermalizing
behaviours in the same model under different initial conditions is linked to
the discussion of `rare' nonthermal states existing in the spectrum. We also
propose a method to predict the collapse and revival time scales and how they
scale with the number of particles in the condensate. We use a sudden quench to
drive the system to non-equilibrium and hence the theoretical predictions given
in this paper can be probed in experiments.
