For atoms or ions that are isoelectronic, the number of protons determines the size. Another isoelectronic series is P 3–, S 2–, Cl –, Ar, K +, Ca 2+, and Sc 3+ (3 s 23 p 6). Examples of isoelectronic species are N 3–, O 2–, F –, Ne, Na +, Mg 2+, and Al 3+ (all have the electron configuration 1 s 22 s 22 p 6). For consecutive elements proceeding down any group, anions have larger principal quantum numbers and, thus, larger radii.Ītoms and ions that have the same electron configuration are said to be isoelectronic. For example, a sulfur atom (3 s 23 p 4) has a covalent radius of 104 pm, whereas the ionic radius of the sulfide anion (3 s 23 p 6) is 170 pm. Both effects (the increased number of electrons and the decreased Z eff) cause the radius of an anion to be larger than that of the neutral atom ( Figure 1). This results in a greater repulsion among the valence electrons and a decrease in Z eff per valence electron, as valence electrons do contribute to screening albeit to a much lesser degree than the core electrons. Proceeding down the groups of the periodic table, we find that cations of successive elements with the same charge generally have larger radii, corresponding to an increase in the principal quantum number, n.Īn anion is formed by the addition of one or more electrons to the valence shell of an atom. The radius for a cation (Al 3+) is smaller than the neutral atom (Al), due to the lost electrons the radius for an anion (S 2-) is larger than the neutral atom (S), due to the gained electrons.Ĭations with larger charges are smaller than cations with smaller charges (e.g., V 2+ has an ionic radius of 79 pm, while that of V 3+ is 64 pm). Even the removal of a single electron produces a cation that is smaller than the parent atom as more accurate methods of determining Z eff that account for the screening effect of the valence electrons, show that losing a valence electron increases Z eff for any remaining valence electrons. All electrons in the n=3 shell are removed, the remaining electrons occupying smaller shells. For example, the radius of a neutral aluminum atom (1 s 22 s 22 p 63 s 23 p 1) is 118 pm, whereas the ionic radius of an Al 3+ (1 s 22 s 22 p 6) is 68 pm. A cation always has fewer electrons and the same number of protons as the parent atom thus, it is smaller than the atom from which it is derived ( Figure 1). Ionic radius is the measure used to describe the size of an ion. | Key Concepts and Summary | Glossary | End of Section Exercises | Ionic Radii Analyze the electron configuration of salts to determine their magnetism. ![]() | Ionic Radii | Electron Configurations of Ions | Correlate the effective nuclear charge with selected trends in periodic properties.
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