My guess is that you're missing a few teeth...probably on the FRONT sprocket. Either that or it's in really bad shape.
Don't run it any longer until you can have a look at it.
You can see the front sprocket without having to remove the engine casing. Feel the teeth...if you can cut yourself on them, they're worn!
Normal sprocket teeth are dull and thick. Worn teeth get pointy with sharp edges. Check the play in the chain at the rear sprocket...I suspect that ALL three probably need replacing.