A transition element X in its ground state has a unique electronic configuration. Its outermost s -orbital is completely empty, while its penultimate d -orbital is completely filled. The atomic number of the element X is:

Step-by-step solution

The electronic configuration of transition elements generally follows the Aufbau principle, but there are exceptions due to the stability of half-filled and fully-filled subshells. Let us examine the ground state electronic configurations of the elements corresponding to the given atomic numbers: Atomic number 29 (Cu): [ Ar ] 3d¹⁰ 4s¹ (outermost s -orbital has 1 electron) Atomic number 47 (Ag): [ Kr ] 4d¹⁰ 5s¹ (outermost s -orbital has 1 electron) Atomic number 48 (Cd): [ Kr ] 4d¹⁰ 5s² (outermost s -orbital is comp