Trends in ionic radii

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 Irrespective of which set of ionic radii are used, the following trends are observed: 

1. In the main groups, radii increase on descending the group, e.g. u+ = 0.76A, Na+ = l.02A, K+- = l.38A, because extra shells of electrons are added.

2. The ionic radii decrease moving from left to right across any period in the periodic table, e.g. Na+ = 1.02A, Mg2+ = 0.720Aand Al3+ = 0.535 A. This is partly due to the increased number of charges on the nucleus, and also to the increasing charge on the ions. 

3. The ionic radius decreases as more electrons are ionized off, that is as the valency incteases, e.g. Cr2+ = 0.80 A (high spin), Cr3+ = 0.615 A, Cr4+ = 0.55 A, CrH = 0.49 A and Cr6+ = 0.44 A. 

4. The d and f orbitals do not shield the nuclear charge very effectively. Thus there is a significant reduction in the size of ions just after 10d or 14f electrons have been filled in. The latter is called the lanthanide contraction, and results in the sizes of the second and third row transition elements being almost the same. 




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