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ionic radius Half the distance between the ‘centres’ of two ions of the same element. Although no precise measurement can be made of the size of individual ions, in practice various techniques (e.g. X-ray diffraction) can be used to estimate ionic radii in particular crystal structures. Generally, it is found that: (a) within the same group of the periodic table ionic radius increases with increasing atomic number; (b) for elements of the same period (i.e. same horizontal row) that form positive ions, ionic radius decreases with increasing positive charge (reflecting the greater nuclear attraction on the same number of extranuclear electrons), for example, Na+ = 1.02, Mg2+ = 0.72, Al3+ = 0.53, Si4+ = 0.40; (c) for the same reasons, if an element can exist in different valence states, the higher the positive charge the smaller the ion, for example, Mn2+ = 0.82, Mn3+ = 0.65; (d) for elements of the same period forming negative ions, the ionic radius increases with increasing negative charge (due to electronic repulsion).