One-to-one function - how to make a proof

For each function, determine whether the function maps onto the given codomain and whether it is one-to-one. Prove your answers.

d) The sequence a: N-> N whose n'th term is An = 2^n

This is the last one of the problem I did not know. Would An be one-to-one? How could I prove my answer afterwards? Serious help required. Thank you. =]