List of atomic expressions
You can executive the code in mma 10 or above version
EntityClass["WolframLanguageSymbol", "Atomic"]//EntityList
But is not all atomic function,as I know.
Since it's the only "answer" I can see to post (as CW) there is also BooleanFunction as originally pointed out by Sasha.
In version 10 Dispatch tables are atomic.(1)
Array[# -> 2 # &, 5] // Dispatch // AtomQ
True