Which of the following pair represents the incorrect electronic confi…

The correct answer is Option (2) → Ni - 3d⁹ 4s¹

In transition elements the (n−1)d and ns orbitals are very close in energy. Electrons normally fill ns first and then (n−1)d, but a half-filled (d⁵) or completely filled (d¹⁰) subshell has extra stability (exchange energy). So an electron shifts from ns to (n−1)d when that shift produces d⁵ or d¹⁰.

• Option (1) Pd - 4d¹⁰ 5s⁰ — A correct configuration. Palladium (Z = 46) moves both 5s electrons into 4d to complete the 4d¹⁰ subshell: [Kr]4d¹⁰.

• Option (2) Ni - 3d⁹ 4s¹ — The incorrect configuration. Nickel (Z = 28) has 10 electrons outside the argon core. A 3d⁹4s¹ arrangement gives neither a half-filled nor a filled d subshell, so there is no reason for an electron to shift. The correct configuration is [Ar]3d⁸4s².

• Option (3) Cu - 3d¹⁰ 4s¹ — A correct configuration. Copper (Z = 29) is [Ar]3d¹⁰4s¹, not 3d⁹4s², because the filled 3d¹⁰ subshell is more stable.

• Option (4) Cr - 3d⁵ 4s¹ — A correct configuration. Chromium (Z = 24) is [Ar]3d⁵4s¹, not 3d⁴4s², because the half-filled 3d⁵ subshell is more stable.

NCERT (2026-27 edition):
Electronic Configurations of the d-Block Elements – “In general the electronic configuration of outer orbitals of these elements is (n-1)d^1–10ns^1–2 except for Pd where its electronic configuration is 4d¹⁰5s⁰. … However, this generalisation has several exceptions because of very little energy difference between (n-1)d and ns orbitals. Furthermore, half and completely filled sets of orbitals are relatively more stable. A consequence of this factor is reflected in the electronic configurations of Cr and Cu in the 3d series. For example, consider the case of Cr, which has 3d⁵ 4s¹ configuration instead of 3d⁴4s²; … Similarly in case of Cu, the configuration is 3d¹⁰4s¹ and not 3d⁹4s². The ground state electronic configurations of the outer orbitals of transition elements are given in Table 4.1.”