[helenee]

In which choice below are the elements ranked in order of increasing first ionisation energy?

A) P, Cl, S, Al, Ar, Si

B) Ar, Cl, S, P, Si, Al

C) Al, Si, P, S, Cl, Ar

D) Al, Si, S, P, Cl, Ar

I had thought the answer would be D, but apparently, it is B. Could you please help me understand why?

[mjc123]

D is correct. Answer given may just be a typo.

[Enthalpy]

https://www.webelements.com/periodicity/ionis_energy_1/
B) would nearly be the decreasing order, except for P vs S.

[helenee]

So just to clarify, I'm right and the book made a mistake?

Thank you for helping me out!

[crchymist]

I wanted to add that you should ask the teacher if they mean the expected trend without any consideration of half shell stability, or to consider energy level stability, or for actual values. 

We would expect it to be P and then S given the trend and no consideration of the p-subshell being half full.   However when we do consider the half shell stability we put S before P. 

Yet we can't do the same thing when we get to 4+ row elements since we have n-1 d subshells to consider as well.  If you were to start with calcium we see that the movement from s to p subshells/orbitals and so expect that np1 vs ns2 to  be lower in IE yet  as we move down the table the impact of half filled shell stability and transition from s to p orbitals become less impactful.  Take In, Sn,  Sb, Te, I, Xe and we find that it follows the trend of increasing IE left to right. We know that the trends are based off the concept of Zeff, electron shielding and electron penetration.

In a basic or lowest level Chemistry class we may not even mention half-shell 'exceptions' , in a CP class we may only  consider these when doing electron configurations with d-block elements.  In an AP or perhaps an honors course there may be opportunity for a student to discuss their response.