When sodium (Na) and fluorine (F) combine, they form sodium fluoride (NaF).

Here's why:

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

* Fluorine (F) is a nonmetal in Group 17, meaning it has a -1 charge when it forms an ion.

* To form a neutral compound, the charges must balance. Therefore, one sodium ion (Na+) combines with one fluoride ion (F-) to form NaF.