 Finite subsets do not define a topology on an infinite set

On an infinite set, the family consisting of all finite subsets and the whole set need not be closed under arbitrary unions. On $\mathbb{Z}$, the union of the singleton subsets of $2\mathbb{Z}$ is infinite and proper, so this family is not a topology. It must not be confused with the cofinite topology, whose proper open sets have finite complements.