Suppose the answer is "Item" although "Atom" also seems plausible, but cannot properly justify either!! Grateful to anybody who can ease my frustration!
D**n - thanks Caravaggio, of course you are correct!! 'Noel' - my only excuse is I typed it very quickly and for some reason had the 'O' missing in my mind - think I was still thinking about the H 2 O post earlier!!