In the above photo, it shows sulfur atom and sulfide ion. In sulfur atom, it has an electronic configuration of (2.8.6). This means that it is not stable as the valance shell only has 6 electrons and to make it stable, it must have 8 electrons. Therefore, in order to make it stable, it needs to gain 2 more electrons to make it 8 electrons to be stable. When it gain 2 electrons, the electronic configuration becomes (2.8.8) and it becomes a sulfide ion. As the overall is negatively charge, it is S2-.