Another possibility is that only a portion of one molecule is transferred to the other molecule. This type of reaction occurs, for example, in redox and acid-base reactions. In redox reactions, the transferred particle is an electron, whereas in acid-base reactions it is a proton.
Psychology is the study of mind and behavior. It is an academic discipline and an applied science which seeks to understand individuals and groups by establishing general principles and researching specific cases.
Abnormal psychology is the study of abnormal thoughts, feelings and behaviors. Abnormal thoughts, feelings and behaviors may or may not be part of a larger mental illness, or psychopathology. But, psychologists who study abnormal psychology usually work to treat people who have some type of mental illness, even if it's just a temporary case of the blues.