The formula for copper(II) nitrate is Cu(NO₃)₂. Here's how we arrive at that:

* Copper(II) indicates the copper ion has a +2 charge (Cu²⁺).

* Nitrate is a polyatomic ion with the formula NO₃⁻ and a -1 charge.

* To balance the charges, we need two nitrate ions (2 x -1 = -2) to match the +2 charge of the copper ion.

Therefore, the formula is Cu(NO₃)₂.