Forward map is:
map([
 hydrogen -> 
   h, 
 helium -> 
   he, 
 lithium -> 
   li, 
 beryllium -> 
   be, 
 sodium -> 
   na])
Reverse map is:
map([
 h -> 
   hydrogen, 
 he -> 
   helium, 
 li -> 
   lithium, 
 be -> 
   beryllium, 
 na -> 
   sodium])