there is some kind of priorities for the elements in propositional logic ?for example : p q  r , given this ,we there may be two options (p q)  r  OR p  (q  r) , which one is the correct ?
Operator precedence in propositional logic
logic;notation;propositional logic
If you look at formal definitions of the syntax of propositional logic, you will find that$\qquad p \land \lnot q \to r$is not a proper sentence; parentheses are needed to avoid exactly the ambiguity you mention.Operator precedences can be used for implicit parenthesisation. You seem to be asking if there are agreed-upon operator precedences in logic.I don't think formal logics contains this concept; formal grammars just do not lend themselves to model precedences (or any ambiguity) very well. In practice (by which I mean both blackboard writing and implemented logic parsers), we do use precedences; usual conventions include$\lnot$,$\land$,$\lor$,$\implies$,$\iff$in decreasing order of precedence. Using these, your example is equivalent to$\qquad (p \land (\lnot q)) \to r$.David's warning is apt, though: if you want to be clear, don't rely on implicit precedences. Typesetting can help -- you can e.g. group terms with spacings -- but in case of doubt, just put the parentheses. In a larger body of work, you can also state your convention once and safe symbols afterwards, provided you stick to your own rules.