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Fluoride ion, whose electron cloud is not easily distorted (like the shape of a golf ball) is said to be hard. Hardness is associated with high electronegativity and/or small atomic radius. Fluorine has high electronegativity (EN = 4.0) and a small atomic radius. | |
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Iodide ion, whose electron cloud is readily distorted (like the shape of a foam ball) is said to be soft. Softness is associated with low electronegativity and/or large atomic radius. Iodine has moderate electronegativity (EN = 2.5) and a large atomic radius. |
