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
Are there any others?
Others mentioned in the comments
Internal`Bag System`Utilities`HashTable System`RawArray BooleanFunction Dispatch (* since v10 *)