Here's why:

* Sodium (Na) is a metal in Group 1 of the periodic table.

* Group 1 elements have one valence electron (electron in the outermost shell).

* To achieve a stable electron configuration (like a noble gas), sodium atoms lose their single valence electron.

* When an atom loses an electron, it becomes positively charged, forming a cation.

* The loss of one negative charge (the electron) leaves the sodium atom with a +1 charge.

Therefore, the correct formula for a sodium ion is Na⁺.