Question

The ions O²⁻, F⁻, Na⁺, Mg²⁺ and Al³⁺ are isoelectronic. Their
ionic radii show
(a) a decrease from O²⁻ to F⁻ and then increase from Na⁺ to
Al³⁺
(b) a significant increase from O²⁻ to Al³⁺
(c) a significant decrease from O²⁻ to Al³⁺
(d) an increase from O²⁻ to F⁻ and then decrease from Na⁺ to
Al³⁺

Answer 1

The ions O²⁻, F⁻, Na⁺, Mg²⁺ and Al³⁺ are isoelectronic. Their  ionic radii  (c) a significant decrease from O²⁻ to Al³⁺

Explanation:

Anion:

When an atom gain an electron anion is formed.

X + e⁻   →  X⁻

Cation:

When an atom lose an electron cation is formed.

X  →   X⁺ + e⁻

The isoelectronic species with greater atomic number have smaller ionic radii because of greater effective nuclear charge.

                       r ∝ 1/Zeff

As the negative charge increases ionic radii increases while in case of cation ionic radii decreases because of removal of electron.

So in given species the ionic radii decreasing order is,

O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³