This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

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The radii of atoms and ions are defied by the maximum radial charge densities of their outermost electron shells. Their sizes vary and are difficult to quantify and measure.

Table 3131. Periodic table for ionic radii (pm) of chemical elements (Atomic Radius = AR, Covalent Radius = 0, positively and negatively charged Ionic Radius = +/-, High Spin = HS, Low Spin = LS, Square Planar = SQ, and Coordination Number 2 = CN2; … Coordination Number 14 = CN14 for the ions with the same charges).

        * The common ionic states are in green.

The size of an atom or ion depends on the size of the nucleus as well as the number of electrons. In general, atoms with more electrons have larger radii than those with fewer electrons.

The magnitude of ionic radii in the ionic crystals is mainly influenced by the following factors:
        i) Coordination number.
        The coordination number is defined as the number of atoms that surround a particular atom or ion in the structure. With the increase of coordination number, the inter-ionic distance and thus the ionic radius increase because each ion with a greater coordination number is surrounded by a greater number of ions and thus results in less attraction between the ions.
        ii) Radius ratio.
        Radius ratio influences the magnitude of ionic radii.
        iii) Covalent character.
        Covalent character in the ionic bond existing between the cation and anion decreases the inter-ionic distance, resulting in a smaller ionic radius.

Other factors that influence the ionic radii are:
        iv) Ionic radii increase for a given coordination number with increasing Z within a periodic group.
        v) Ionic radii decrease with increasing nuclear charge for isoelectronic ions.
        vi) Ionic radii decrease with increase of ionic charge for the same Z. Therefore, as the charge on the ions becomes more positive, there will be fewer electrons and the ions will have smaller radii. On the other hand, as the charge on the ions becomes more negative, there will be more electrons and the ions will have larger radii (see Table 3131).
        vii) Larger ionic radius for high-spin ions than for low-spin ions of the same species and charge.
        viii) As the atomic number increases in any given column of the Periodic Table, the numbers of protons and electrons increase and thus the size of the atom or ion increases.
        ix) The ionic radii decrease with increase in atomic number as indicated in Figure 3131.

Figure 3131. Ionic radii of 3d elements.