A name can have digits but not entirely made of digits, because then it would be read as a number. Similarly a name can have periods, but can't be made entirely of periods.
Use of Single Quotation Mark
LISP evaluates everything including the function arguments and list members.
At times, we need to take atoms or lists literally and don't want them evaluated or treated as function calls.
To do this, we need to precede the atom or the list with a single quotation mark.