* Sodium (Na) is in Group 1 of the periodic table, meaning it has a +1 charge as an ion (Na⁺).

* Iodine (I) is in Group 17 of the periodic table, meaning it has a -1 charge as an ion (I⁻).

To form a neutral ionic compound, the charges must balance. Since sodium has a +1 charge and iodine has a -1 charge, one atom of each is needed to create a neutral compound.