The problem does not occur with Netscape Navigator which correctly sets the window.opener.closed property to true if the window from which the Periodic Table was opened is subsequently closed.
Problems may be experienced when running the enthalpy definition quizzes on a network.
The cause of this is highly technical and I'm not sure I fully understand it myself.
There are two ways around this problem.
Avoid accessing the quiz via a hyperlink, instead (i) open the quiz using the standard File-Open command or (ii) by dragging and dropping the file on to the icon for Netscape Navigator or Internet Explorer.