I can create the single-element list (2) in all of the following ways:
'(2)
(list 2)
(cons 2 nil)
However, why does the following give an error?
(2 . nil)
Is there no such thing as the "scalar null" value in scheme? Or is there an alternate way to do that?
(2 . '()) does indeed represent the singleton list '(2): the latter is shorthand for the former. The problem is that, if you use it in the same context that you would use (cons 2 '()) in, you get an error. Why? The same error you would get if you tried to evaluate (2)! That means to call 2 as a function, passing it no arguments.
If instead you want to just create such a list, without calling it, you do the same thing you would with the shorthand: quote it.
'(2 . ())