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What does isoelectronic mean in the context of atoms and ions?
Isoelectronic atoms and ions are those that have the same number of electrons but may have different numbers of protons.
For example, ions or atoms of different elements can have the same electron configuration and thus be isoelectronic.
How do you predict the relative sizes of isoelectronic atoms and ions?
Isoelectronic species with more protons (higher atomic number) will have a smaller radius because the increased nuclear charge pulls the electrons closer to the nucleus.
Isoelectronic species with fewer protons (lower atomic number) will have a larger radius due to weaker nuclear attraction on the same number of electrons.
In short, as nuclear charge (Z) increases, the ionic radius decreases.
Predict the relative sizes of O²⁻, F⁻, Na⁺, Mg²⁺, and Al³⁺, all of which are isoelectronic with Ne (neon).
These ions have the same electron configuration as Ne (1s² 2s² 2p⁶).
Al³⁺ has the greatest nuclear charge (13 protons) and thus has the smallest radius.
Mg²⁺ has 12 protons and is larger than Al³⁺.
Na⁺ has 11 protons and is larger than Mg²⁺.
F⁻ has 9 protons, and O²⁻ has 8 protons, making them the largest in this series, with O²⁻ being the largest due to having the fewest protons pulling on the same number of electrons.
How can you summarize the trend in size for isoelectronic atoms and ions?
For isoelectronic species, ionic radius decreases as nuclear charge (protons) increases.
This means that cations (positive ions) are smaller than anions (negative ions) when they are isoelectronic because cations have more protons attracting the same number of electrons.
