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Which of the following is an INCORRECT match between an element's electronic configuration (or atomic number) and its group/period in the periodic table?

1. Z = 48, Group IIB, Period 5
2. [Xe] 4f⁷ 5d¹ 6s², Group IIIB, Period 6
3. [Rn] 6d² 7s², Group IVB, Period 7
4. Z = 56, Group IIA, Period 6

Correct answer: [Xe] 4f⁷ 5d¹ 6s², Group IIIB, Period 6

Solution

Z=48 is Cd: [Kr]4d¹⁰ 5s², Group 12 (IIB), Period 5. Correct. [Xe]4f⁷ 5d¹ 6s² is Gd (Z=64), Period 6. However, Gd is a lanthanide (f-block element). In IUPAC classification, Group IIIB (Group 3) in Period 6 corresponds only to La (Z=57). The lanthanides (Ce-Lu) are f-block and placed in a separate row. Calling Gd a Group IIIB element is INCORRECT. [Rn]6d² 7s² is Rf (Z=104), Group IVB, Period 7. Correct. Z=56 is Ba: [Xe]6s², Group IIA, Period 6. Correct.

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