manxman, I wasn't being that specific. I was just going for elements on the periodic table. It's like chemistry class here today and you're doing well.
"tin" is too short according to the rules and "cadmium" doesn't work. Look at the available letters because I believe you haven't used all of them.
Bob, I borrowed this puzzle from our illustrious leader - Pandora. I'd like to claim the credit but... The hardest part is cracking the category. Once you work that out the rest should come a little easier.