In the formula XSO4 what element is X?

If meaning an element of the Periodic Table, element "X" does not exist as of now. However, there is an element that is symbolized as "Xe", which is also known as Xenon.
There is also an element "Y" (Yttrium), as well as "V" (Vanadium), "U" (Uranium), and "W" (Tungsten).
- Fun fact: There is no periodic element that contains the letter j. -