How many atoms of each element are needed to create an ionic compound of strontium selenide

Step 1:
           Calculate the ionic charges on both atoms:

As Strontium belongs to Group-II in periodic table so it will tend to loose two electrons and form Sr⁺² , i.e.

                                        Sr    →     Sr⁺²  +  2 e⁻

While, Se belonging to Group-VI will tend to gain two electrons and form Se⁻², i.e. 

                                       Se  +  2 e⁻    →    Se⁻²

Step 2:
           Multiply the coefficient of each element by the charge of other element.

                                    Se⁻² × 2  =  2 Se⁻²           ∴ where 2 is charge on Sr
And,
                                    Sr⁺² × 2  =  2 Sr⁺²             ∴ where 2 is charge on Se

So, the formula comes out to be,

                                                  Sr₂Se₂

Or divide both coefficients by 2 to have a simplest form. i.e...

                                                    SrSe

Result:
           One One atoms of each element are needed to create an ionic compound of Strontium Selenide.