The chemical formula for the compound formed between sodium (Na) and selenium (Se) is Na₂Se. Here's why:

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

* Selenium (Se) is in group 16 of the periodic table, meaning it has a -2 charge as an ion (Se⁻²).

To form a neutral compound, you need two sodium ions (2 x +1 = +2) to balance the charge of one selenium ion (-2). This gives you the formula Na₂Se.