showing non-isomorphism of groups

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How do I prove that there is no isomorphism between $\Bbb Z$ under addition and $\Bbb Q$ under addition?

They both are infinite order. I thought they might be isomorphic.

Help would be appreciated!

Thanks in advance!

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Hint: If $r$ is any element of $\mathbb{Q}$, there is an $x$ such that $x+x=r$.