Lepton (Level 19)

 A lepton is an elementary particle and a fundamental constituent of matter. The most well-known of all leptons is the electron which governs nearly all of chemistry as it is found in atom and is directly tied to allchemical properties. Two main classes of leptons exist, charged leptons (also known as the electron-like leptons), or neutral leptons (better known as neutrinos). Charged leptons are very common, and can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed.     
 
http://en.wikipedia.org/wiki/Lepton