My Chemistry teacher is at it again. Today, I cautiously approached his office and stopped in the doorway instead of walking right in - and it's a good job I did. There he was again - bent over on the carpet, arranging his Periodic Table element flashcards in some new and completely unscientific manner, taking up almost the entire floor.
Under his breath, he muttered, "Gargh, I haven't left enough room at the top..." and in his hands he held two flashcards whose faces I couldn't quite see. At this point he became suddenly aware of my presence, stood up abruptly, and concealed these two flashcards behind his back.
I opened my mouth to speak but he beat me to it. "You have another question about the classwork, don't you?" He smiled. "Tell you what - you can ask your question, but only if you can first tell me which two elements I am holding in my hand..."
TASK: Considering the arrangement of flashcards below, which two elements are missing from the pattern? Where should they both be placed in relation to the other cards on the floor?