Here's why:

* Atoms are made of protons, neutrons, and electrons.

* Protons have a positive (+) charge.

* Electrons have a negative (-) charge.

* Neutrons have no charge (they are neutral).

* In a non-ionized atom, the number of protons (positive charges) is equal to the number of electrons (negative charges).

Since the positive and negative charges balance each other out, the overall charge of the atom is neutral or zero.