* Ionic compounds are formed by the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions).

* Nickel (Ni) is a metal and tends to lose electrons to form a positively charged ion (Ni²⁺).

* Sulfate (SO₄²⁻) is a polyatomic ion with a negative charge.

Therefore, nickel sulfate is formed by the electrostatic attraction between the Ni²⁺ cation and the SO₄²⁻ anion, making it an ionic compound.