Question

For an isoelectronic series of ions, what is the ion that is always the smallest?

Answer 1

Hint: An ion is a net electrically charged particle, atom, or molecule. By convention, the electron's charge is regarded as negative. By convention, an electron's negative charge is equal to and opposite that of charged proton(s) deemed positive. An ion's net charge is non-zero because its total number of electrons is greater than its total number of protons.

Complete answer:
An anion is a negatively charged ion with more electrons than protons, whereas a cation is a positively charged ion with less electrons. Cations and anions are attracted to one other by their opposing electric charges and easily form ionic compounds. Isoelectronicity occurs when two or more molecules have the same structure (positions and connectivities among atoms) and electron configurations, but differ in which particular components are present at certain places in the structure.
Atoms or ions with the same amount of electrons are known as isoelectronic species. The amount of electrons in isoelectronic species would be the same, but the elements would be different. In other terms, isoelectronic species refers to ions and atoms that have the same amount of electrons.
The smallest ion in an isoelectronic sequence of ions is always a cation with a large positive charge or an anion with a small negative charge.
Let's have a look at an example. Take a look at the following ions:
$$Ar,S^{2-},K^{+},C{l}^{-},C{a}^{2+}$$
$C{a}^{2+}$ has the lowest size because it is a cation with a large positive charge magnitude.
The value of $Z_{eff}$ rises as more electrons are extracted from Ca (number of protons in nucleus increases).
As a result, the electron cloud is dragged close to the nucleus.
As additional electrons are supplied, the biggest ion will be $S^{2-}$.
The nucleus' influence diminishes, while the electron cloud grows.
As a result, the order of radii is: $$S^{2-}>C{l}^{-}>Ar>K^{+}>C{a}^{2+}$$

Note:
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.)