Which one among the following is the correct arrangement in increasing order of ionic radii of O²⁻, F⁻, Na⁺, Mg²⁺ ?
[amp_mcq option1=”Mg²⁺ < Na⁺ < F⁻ < O²⁻" option2="O²⁻ < F⁻ < Na⁺ < Mg²⁺" option3="Mg²⁺ < Na⁺ < O²⁻ < F⁻" option4="O²⁻ < F⁻ < Mg²⁺ < Na⁺" correct="option1"]
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UPSC Geoscientist – 2024
For isoelectronic species, the ionic radius decreases as the nuclear charge (atomic number) increases. A higher nuclear charge exerts a stronger attraction on the same number of electrons, pulling them closer to the nucleus and reducing the ionic size.
The atomic numbers are: Oxygen (O) = 8, Fluorine (F) = 9, Sodium (Na) = 11, Magnesium (Mg) = 12.
The effective nuclear charges are approximately +8 for O²⁻, +9 for F⁻, +11 for Na⁺, and +12 for Mg²⁺.
As the nuclear charge increases (from +8 to +12), the attraction on the 10 electrons increases, leading to a decrease in ionic size.
Order of increasing nuclear charge: O²⁻ (+8) < F⁻ (+9) < Na⁺ (+11) < Mg²⁺ (+12). Order of decreasing ionic radii: O²⁻ > F⁻ > Na⁺ > Mg²⁺.
Order of increasing ionic radii: Mg²⁺ < Na⁺ < F⁻ < O²⁻.