White phosphorus
Other names: tetraphosphorus
P4
28/50 
common
White phosphorus is a highly unstable allotrope of elemental phosphorus that ignites violently and spontaneously in the presence of air. Despite this fact, and its ubiquitous use in incendiary munitions of dubious international legality, this terrifying molecule is also a necessary precursor for any phosphorus fine chemicals – a dizzyingly broad swathe of reagents with applications ranging from lithium battery electrolytes to synthetic herbicides. White phosphorus is even used to synthesize the food-grade phosphoric acid used as a flavoring and antimicrobial agent in soft drinks such as Coca-Cola.
Of course, the real reason P4 is on this list of trading cards is because I spent the better part of my PhD attempting to synthesize it by molten-salt electrolysis.