The formula for nickel(II) dichromate is NiCr₂O₇. Here's how to determine it:

* Nickel(II): The Roman numeral II indicates that nickel has a +2 charge (Ni²⁺).

* Dichromate: The dichromate ion has a -2 charge (Cr₂O₇²⁻).

* Balancing Charges: To form a neutral compound, one nickel(II) ion (Ni²⁺) combines with one dichromate ion (Cr₂O₇²⁻).

Therefore, the formula is NiCr₂O₇.