Ionization energy is defined as the energy required to remove the electron requiring the least amount of energy in the atom.
When valence electrons are being removed, the ionization energy will be quite low. However, once there is a spike in the number (In this case, IE6 starts at 8496 and jumps to 27110 in IE7), that indicates that a core electron is being removed.
Since there are 6 ionization energies that are relatively low before the spike in energy in IE(7), it can be inferred that this period 3 element has 6 valence electrons.
Because of this, we simply have to go from left to right in period 3 six times. (Make sure to skip over the transition metals if you're counting past period 3, since their valence electrons vary)
Sulfur is the mystery element
Good luck! If you have any questions, just ask! :))
