Why is tin IV selenide SnSe2?

Answer 1

Tin(IV) has a positive four charge.

Selenide usually forms a 2- ion.

Ionic compounds need a neutral, balanced charge.

With the above information, the rest is pretty much just math.

Tin - Sn(+4)

Selenide - Se(-2)

So you need two of Se to balance the +4 with a -4.

= SnSe2

The answer above is OK, however if you are familiar with the idea of oxidation states SnIV is tin in its 4th oxidation state. Selenium has a common oxidation stae of -2 and once again you can do the math.

The bonding in SnSe2 is not ionic so using oxidation states is preferable.