Here's why:

* Na represents the element Sodium.

* Se represents the element Selenium.

* The subscript 2 indicates there are two Sodium atoms for every one Selenium atom.

Since Sodium is a Group 1 element, it forms a +1 cation (Na⁺). Selenium is a Group 16 element and typically forms a -2 anion (Se²⁻). To balance the charges, you need two Sodium ions (2 x +1 = +2) to combine with one Selenide ion (-2).