What is the highest principal energy level for an electron in an atom of sulfur in the ground state?

The number of valence electrons from the electron configuration, write 1s², 2s² 2p⁶, 3s² 3p⁴.

The "big" numbers are the principal quantum numbers (n) which correspond to the major energy levels, and sub-scripted numbers are the number of electrons in the sub-energy level.

For sulfur, the highest principal quantum number is 3, and there are 6 electrons in the 3s² and 3p⁴ sublevels.