Answer:
2. (C) K⁺; 3. (E) Hg⁺; 4. Hg⁺
Explanation:
We must first write the electron configurations of the different species.
(A) Fe²⁺
Fe: [Ar]4s²3d⁶
Fe²⁺: [Ar]3d⁶
When removing electrons from a transition metal ion, you remove the s electrons first.
(B) Cl
Cl: [Ne]3s²3p⁵
(C) K⁺
K: [Ar]4s
K⁺: [Ar]
(D) Cs
Cs: [Xe]6s
(E) Hg⁺
Hg: [Xe]6s²4f¹⁴5d¹⁰
Hg⁺: [Xe]6s4f¹⁴5d¹⁰
2. K⁺ has a noble gas configuration
3. Hg⁺ has electrons in f orbitals.
4. The electron configuration of Au is [Xe]6s4f¹⁴5d¹⁰, not [Xe]6s²4f¹⁴5d⁹, because a filled d subshell is more stable than a filled s subshell.
Thus, Hg⁺ is isoelectronic with Au.
