Question

Which of the following make an isoelectronic pair: $C{{l}^{-}},~{{O}^{2-}},~F,~C{{a}^{2+}},~F{{e}^{3+}}$?

Answer 1

Each electron is described as travelling freely in an orbital in an average field generated by all other orbitals in electronic setups. Configurations are characterised mathematically by Slater determinants or configuration state functions. According to quantum physics, each electron configuration has a level of energy associated with it, and electrons may migrate from one configuration to another by emitting or absorbing a quantum of energy in the form of a photon under specific conditions.

Complete answer:
Isoelectronicity is a phenomenon that occurs when two or more molecules have the same structure and electron configurations but differ in the specific components present at different points in the structure.
Valence isoelectronicity is a word used to describe this concept. Definitions aren't always as rigorous, requiring the identification of the total electron count and, with it, the whole electron configuration in some cases. Typically, definitions are wider, allowing for various quantities of atoms in the species under comparison.
The concept's usefulness rests in recognising closely related species as pairs or series. Because characteristics of isoelectronic species are anticipated to be consistent and predictable, recognising a molecule as isoelectronic with one that has already been described provides hints about probable properties and reactions. (Reactivity can be affected by differences in characteristics such as the electronegativity of the atoms in isoelectronic species.)
In $C{{l}^{-}}$, total number of electrons present is 18 (17 + 1)
In ${{O}^{2-}}$, total number of electrons present is 10 (8 + 2)
In F, total number of electrons present is 9
In $C{{a}^{2+}}$, total number of electrons present is 18 (20 - 2)
In $F{{e}^{3+}}$, total number of electrons present is 23 (26 - 3)
"Same electronic structure" is what is meant by isoelectronic. We may say "identical number of valence electrons" for atoms. If an atom, an ion, or two ions have the same number of electrons, we may infer that they will be organised in the same shells and subshells. Both the calcium ion and the chloride ion contain 18 electrons.
Hence $C{{l}^{-}},C{{a}^{2+}}$are isoelectronic in nature.

Note:
Isoelectronicity in diatomic molecules is best illustrated using molecular orbital diagrams, which demonstrate how orbital mixing in isoelectronic species leads to similar orbital combinations, and therefore bonding. Polyatomic molecules can also be found in more complicated compounds. The amino acids serine, cysteine, and selenocysteine, for example, are all isoelectronic. They are distinguished by the presence of a particular chalcogen at a specific position in the side-chain.