Periodic trends: same period shares shells; same group shares valence electrons Steps: - Elements in the same period have the same number of electron shells, shown by the integer part of the configuration (e.g., 2 for second period). - Elements in the same group have the same number of valence electrons, shown by the decimal part. - X and Y in same period requires matching integer parts for X and Y. - Y and Z in same group requires matching decimal parts for Y and Z; check options for both conditions. Why B is correct: - Option B has X (2.4) and Y (2.7) both with 2 shells (same period) and Y (2.7) and Z (2.7) both with 7 valence electrons (same group, per group definition of identical outer electron count). Why the others are wrong: - A: Y (2.7) and Z (2.4) have different valence electrons (7 vs. 4), violating same group. - C: Y (2.8) and Z (2.7) have different valence electrons (8 vs. 7), violating same group. - D: Y (2.7) and Z (2.4) have different valence …