It's strange. If Graph is an atom than why we can browse Graph in FullForm?

I believe this is because certain critical operations that Mathematica must do, such as serializing (think Compress, using WSTP or WXF or just Save) an expression, either requires (1) special and separate treatment of each and every atomic type or (2) a means to convert an atomic expression into an equivalent compound one, and back.

(1) is unfeasible. There are too many atomic types. Many exist for performance reasons. Thus most of the time (2) is used.

See here:

• https://mathematica.stackexchange.com/a/96604/12
• https://mathematica.stackexchange.com/a/97332/12

and other answers in the same thread.

Don't be fooled by Graph having a compound form. It really is an atom, and its compound representation would only be equivalent to its true internal representation in an ideal world. It's fairly common for bugs in Graph to reveal differences between the two ...

E.g. Graph3D[KaryTree[5], PlotTheme -> "CoolColor"] fails. Graph3D[Uncompress@Compress@KaryTree[5], PlotTheme -> "CoolColor"] works. I used Compress to effectively cycle the graph through its compound representation.

It's strange. If Graph is an atom than why we can browse Graph in FullForm?

In[57]:= FullForm@Graph[{1->2,3->2}]
Out[57]= Graph[List[1,2,3],List[DirectedEdge[1,2],DirectedEdge[3,2]]]

On the other hand, we can't change Head for Graph symbol also. So, is it Graph treated in a special way in Wolfram Language? If it's true then It may explain why TreeForm behaves in such a way.

In[58]:= FullForm[ Apply[f,Graph[{1->2,3->2}]] ]
Out[58]= Graph[List[1,2,3],List[DirectedEdge[1,2],DirectedEdge[3,2]]]

In[59]:= FullForm[ Apply[f,Graphics[{Circle[{0,0}],Disk[{2,2}]}]] ]
Out[59]= f[List[Circle[List[0,0]],Disk[List[2,2]]]]