Atomic number of sulfur is 16. The electronic configuration of sulfur (S) can be written as
. So, inorder to reach the stable octet configuration the valence shell tends to accept two electrons to form sulfide, a divalent anion.
This can be represented as,
![S + 2e^(-)-->S^(2-)](https://img.qammunity.org/2019/formulas/chemistry/high-school/5k7gxfb05onobc1xx0r17ljao6wr36jz4e.png)
Therefore, the charge on sulfide ion
is -2 as sulfur atom tries to get a stable octet configuration by accepting two electrons giving a negative charge to the ion formed.