Why is As2S3 Named Arsenic Trisulfide?

[zachem62]

Homework Statement

I thought that because Arsenic is not a non-metal, you don't use the number prefixes when naming compounds that are not non-metals. So my guess for As2S3 was Arsenic sulfide. But the actual answer is Arsenic trisulfide. Why is this?

Homework Equations

No relevant equations

The Attempt at a Solution

Arsenic sulfide

 

[Bystander]

Why is this?

Distinguishes it from the di- and penta-sulfides, AsS and As₂S₅.