Identifying the period of element Z (atomic number 31) in the periodic table Steps: - Calculate cumulative elements up to period 3: 2 (period 1) + 8 (period 2) + 8 (period 3) = 18 elements. - Period 4 spans atomic numbers 19 to 36, as it includes the 4s, 3d, and 4p subshells (2 + 10 + 6 = 18 elements). - Atomic number 31 falls between 19 and 36. - Thus, element Z (gallium) is in period 4; the Z⁻ ion detail does not alter its neutral position. Why C is correct: - Periods in the periodic table are defined by the filling of electron shells corresponding to the principal quantum number; atomic numbers 19–36 fill the n=4 shell. Why the others are wrong: - A. Group iii: Gallium is in group 13 (IIIA in older notation), but the option specifies group without "A," and the question focuses on overall location including period. - B. Group V: Group V (15) in period 4 is arsenic (atomic number 33), not 31. - D. Period 5: Period 5 spans atomic …