Choose the element with the highest ionization energy element: Na, Mg, Al, P, S

Question

In my book: ionization energy increase from left to right and bottom to top.

Na<Mg<Al<P<S
S is highest ionization energy?
my instructor review this problem and his answer is P is a highest ionization. Please explain

Thank you for your help

Answer 1

Your book is not quite right. See this chart http://users.humboldt.edu/rpaselk/ChemSupp/HSU-PT/HSU_PerTab_Y_IE.jpg
so ionization energy increases from the bottom left towards the upper right.That explains P and S. When looking at "trends", one has to be careful in making rigid rules.

http://www.emsb.qc.ca/laurenhill/science/Trends2/ionization%20energy.jpg