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| Ionic Radius |
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| It is defined as the distance from the centre of the nucleus upto the last shell containing the electrons. |
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| Ionic radii can be obtained from the dimensions of the unit cell which are obtained from x-ray studies. |
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| It has been noticed that the interionic distances obtained from the calculated value are in good agreement with the observed values (X-ray studies). |
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| Ionic radii of common ions |
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| Calculated sum of ionic radius and observed inter ionic distance |
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| Along a group |
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| Ionic radii increases down a group. |
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| Na+ < K+ < Rb+ < Cs+ |
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| F- < Cl- < Br- < I- |
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| This is because |
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 the number of shells increase. |
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screening effect increases. |
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| Along a period |
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| For isoelectronic ions (ions with the same number of electrons) belonging to the same period greater the nuclear charge smaller is the ionic size. |
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| Al3+ < Mg2+ < Na+ |
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| Sizes of cations and anions versus corresponding atoms |
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Anions are larger than the neutral atoms. The effective nuclear charge decreases and the electron cloud expands. |
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| Cl- > Cl |
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Cations are smaller than the neutral atom because the effective nuclear charge increases. |
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| Na > Na+ |
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