What letters aren't on the periodic table?
J and Q are the only letters left out of the periodic table, and this answer would have been different just 6 months ago. Element 114, which was just called Ununquadium (basically 114ium) had the symbol Uuq. This changed on May 30th, 2012 when it was officially name Fleovium and given the symbol Fl. But J and Q aren't completely ignored when it comes to atoms.
J is the symbol for an atoms total angular momentum, which is pretty important.
|This is a term symbol. L is the orbital angular momentum, J is the total angular momentum, and S is the spin.|
Q is an important symbol in rotational spectroscopy. Oddly enough for this discussion it is seen when the change in total angular momentum, J, equals zero. It seems that J and Q are destined to be left alone, together.
|This diagram shows the possible spectroscopic transitions. Notice that the P branch is formed when the change in J is equal to -1, the R branch is formed when the change in J is equal to +1, and the Q branch is formed when the change in J is equal to zero.|