* Sodium (Na) is a metal and tends to lose one electron to achieve a stable electron configuration. This results in a positively charged sodium ion (Na+).

* Iodine (I) is a nonmetal and tends to gain one electron to achieve a stable electron configuration. This results in a negatively charged iodide ion (I-).

The strong electrostatic attraction between the positively charged sodium ion and the negatively charged iodide ion forms the ionic bond in sodium iodide.