Complex sentence: It is considered an "atypical" antipsychotic because of its display of serotonin receptor antagonism, similar to other atypical antipsychotics whereas the older typical antipsychotics are primarily dopamine antagonists.
Split into simpler sentences: It is considered an "atypical" antipsychotic because it displays serotonin receptor antagonism, similar to other atypical antipsychotics. The older typical antipsychotics are primarily dopamine antagonists.