To calculate the oxidation number for sulfur in Li2S, we can use the following rules:

1. The oxidation number of an element in its elemental form is 0.

2. The oxidation number of a Group 1 metal is +1.

3. The oxidation number of a Group 2 metal is +2.

Using these rules, we can determine the oxidation number for sulfur in Li2S as follows:

- The oxidation number of lithium (Li) is +1.

- There are two lithium atoms in Li2S, so the total oxidation number of lithium in Li2S is +2.

- The overall charge of Li2S is 0, so the oxidation number of sulfur (S) must be -2.

Therefore, the oxidation number for sulfur in Li2S is -2.