Abelian group in module

How to say every abelian group G is a module over the ring of integers I?

Let G be an abelian group, the operation in G being denoted by+ and the identity element of G by 0. For any integer n and for any element an of G  we define na in several ways:

If n is a positive integer, we define na=a+a+.....nterms.if n=0, we define 0a=0 where 0 on the right-hand side is the identity of G.

If n is a negative number, we can say n=-m where m is +ve integer, we define  (-m)a=-(ma) where -ma denoted the inverse of ma in G.It can be easily seen that -(ma)=m(-a).