The letter "J" is the only one not found on the periodic table.
In some countries (e.g., Norway, Poland, Sweden, Serbia, Croatia), the element iodine is known by the name jod. However, the periodic table still uses the IUPAC symbol I for the element.
About The Element Ununtrium
There was speculation the newly discovered element 113 (ununtrium), might get a permanent name starting with a J and element symbol J. Element 113 was discovered by the RIKEN collaboration team in Japan. However, the researchers went with the element name nihonium, based on the Japanese name for their country, Nihon koku.
The Letter Q
Note that the letter "Q" does not appear in any official element names. Temporary element names, such as ununquadium, contain this letter. However, no element name starts with Q and no official element name contains this letter. Once the final four elements on the present periodic table get official names, there will be no Q on the periodic table. The extended periodic table, which includes undiscovered superheavy elements (atomic numbers greater than 118) would still contain the letter Q in temporary element names.