It's spelled lens, the plural is lenses.
I will have to quote this post in the constant debate between filters for protection or not. I have always maintained that a filter stands more chance of damaging a front element in a drop than it will help protect it.
A hood and your lens cap is much better protection. A filter is thin glass with soft metal surrounding it, and any amount of trauma will bend the metal, and that is likely to break the glass. That broken glass has a good chance of scratching the front element, and in your case I am sorry to hear it did.
The good news is that the scratch will not likely show up in your photographs. I had an 80-200 f/2.8L that I picked up used in ugly condition and it had a big gouge in the front element. I mean deep and big, about the size of a fingernail and definitely measurably deep(I would guess at least 1/64"), and that canyon on my front element didn't show up in any photographs.