Which one of the following is the correct order of ionic radii?

Which one of the following is the correct order of ionic radii?
  1. \(\mathrm{Pr}^{3+} > \mathrm{Gd}^{3+} > \mathrm{Tm}^{3+}\)
  2. \(\mathrm{Pr}^{3+} < \mathrm{Gd}^{3+} < \mathrm{Tm}^{3+}\)
  3. \(\mathrm{Pr}^{3+} > \mathrm{Tm}^{3+} > \mathrm{Gd}^{3+}\)
  4. \(\mathrm{Pr}^{3+} < \mathrm{Tm}^{3+} < \mathrm{Gd}^{3+}\)

Solution

In \(4 f\)-series, as the atomic number increases from \(\mathrm{La}\) to \(\mathrm{Lu}\) the atomic and ionic radii decreases gradually. This decrease in atomic and ionic size from lanthanum to lutetium is due to lanthanide contraction. Hence, the correct order of ionic radii of given element is \(\underset{(101 \mathrm{pm})}{\operatorname{Pr}^{3+}} > \underset{(94 \mathrm{pm})}{\mathrm{Gd}^{3+}} > \underset{(87 \mathrm{pm})}{\operatorname{Tm}^{3+}}\)

Asked in: AP EAMCET 2019 (22 Apr Shift 1)

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