List of atomic expressions

I was surprised to see Graph objects are atomic. Is there a way (through documentation or programmatically) to find all atomic heads?

The ones I know of are:

Integer
Rational
Real
Complex
Symbol
String
SparseArray
StructuredArray
Image (* since v9 *)
Image3D
Graph
ColorProfileData
Association
MeshRegion
BoundaryMeshRegion
LanguageArrayObject
ByteArray
QuantityArray


Are there any others?

Others mentioned in the comments

InternalBag
SystemUtilitiesHashTable
SystemRawArray
BooleanFunction
Dispatch (* since v10 *)
Dataset

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Strangely, SparseArrays are actually not atomic, but just treated for most purposes as if they were. They are the only non-atomic atoms, though, as far as I know. And in addition to your list, there are several other undocumented atomic objects as well, such as the InternalBag, the SystemUtilitiesHashTable, the SystemRawArray, and probably others besides. Several objects that should be atomic (by the standards of the SparseArray) aren't, such as CompiledFunction and LibraryFunction. –  Oleksandr R. Apr 27 '14 at 3:38
The same question on Stack Overflow: (5964469) –  Mr.Wizard Apr 28 '14 at 19:19
What is atomic and what isn't changes with versions. Image isn't atomic in v7 and v8. –  Szabolcs Apr 29 '14 at 16:54
@ChipHurst AtomQ is defined as "an expression which cannot be divided into subexpressions", and has notes such as "You can use AtomQ in a recursive procedure to tell when you have reached the bottom of the tree corresponding to an expression." and "AtomQ gives True for any object whose subparts cannot be accessed using functions like Map". Clearly Association should not be AtomQ by these measures. But as you point out, it is also not NormalQ (which would mean that it behaves like its FullForm), if we had such a thing. AtomQ and NormalQ are currently "mixed together", we should separate them. –  Taliesin Beynon Jul 17 '14 at 18:20
@TaliesinBeynon ... With[{a = Association[1 -> 2]}, Hold[a]]. But it doesn't, there are differences (again, part extraction, pattern matching). Also consider SparseArray, which also has parts, but again doesn't behave identically to its FullForm. SparseArray is also marked as AtomQ for this reason. Perhaps you could consider changing the description of AtomQ in the documentation instead of letting AtomQ return False for associations. What AtomQ really means is a bit complicated, but it's valuable to have it, and changing it would break either consistency ... –  Szabolcs Aug 5 '14 at 14:39

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

The same as association. <|a -> 1|> // AtomQ gives True. –  Yi Wang Jul 14 '14 at 15:55
@YiWang Association` is already mentioned in the Question body above, but yes. –  Mr.Wizard Jul 14 '14 at 23:36