Question

Which set of elements is in the correct order of increasing first ionization energy?

Responses

P, S, Cl
P, S, uppercase C lowercase l

Be, Mg, Ca
uppercase B lowercase e , uppercase M lowercase g , uppercase C lowercase a

F, O, N

F, O, N

Cl, Br, I

Answer 1

None of the listed sequences is in correct order of increasing first ionization energy.

Correct increasing orders:
- P, S, Cl → S < P < Cl (S slightly lower than P due to p‑orbital pairing)
- Be, Mg, Ca → Ca < Mg < Be (IE decreases down group 2)
- F, O, N → O < N < F (O is lower than N because of electron‑pair repulsion)
- Cl, Br, I → I < Br < Cl (IE decreases down the halogen group)

Reason: IE generally increases left→right across a period and decreases top→bottom in a group, with small exceptions where paired electrons lower IE (e.g., N vs O, P vs S).