closed shell

12 LETTERS · STARTS WITH C

closed shell is a valid 12-letter word worth 17 points in Scrabble and 20 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS

C3L1O1S1E1D2S1H4E1L1L1

3 + 1 + 1 + 1 + 1 + 2 + 0 + 1 + 4 + 1 + 1 + 1 =17 PTS

DOUBLE WORD → 34TRIPLE WORD → 51

MEANING

What does closed shell mean?