Here's why:

* Sulfate (SO₄²⁻) is a polyatomic ion, meaning it's a group of atoms that act as a single unit with a charge.

* Sulfur (S) has 6 valence electrons and needs 2 more to achieve a stable octet.

* Oxygen (O) has 6 valence electrons and needs 2 more to achieve a stable octet.

* In sulfate, each oxygen atom shares two electrons with the sulfur atom, forming a double bond. This leaves each oxygen atom with one extra electron, giving it a negative charge (-1).

* There are four oxygen atoms in sulfate, resulting in a total negative charge of -4.

* The sulfur atom, however, is sharing its electrons with the oxygen atoms, and thus has a positive charge of +6.

* The overall charge of the sulfate ion is the sum of the charges on each atom, which is -4 + 6 = -2.

Therefore, sulfate has a negative charge of -2.