The given ions O²⁻, F⁻, Na⁺, and Mg²⁺ are isoelectronic species, meaning they all have the same number of electrons (10 electrons, with the electronic configuration $1s^2 2s^2 2p^6$). 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⁻ > Na⁺ > Mg²⁺. Order of increasing ionic radii: Mg²⁺
Which one among the following is the correct arrangement in increasing order of ionic radii of O²⁻, F⁻, Na⁺, Mg²⁺ ?
Asked in UPSC Geoscientist 2024 · Topic: Inorganic Chemistry (Atomic Structure and Periodicity)
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