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# Arrange the following monatomic ions in order of increasing atomic radii: P$^{3–}$, Cr$^{6+}$, Ca$^{2+}$, Cl$^{-}$, Ti$^{4+}$.

Answer: Cr$^{6+}$ < Ti$^{4+}$ < Ca$^{2+}$ < Cl$^{-}$ < P$^{3–}$
Compare positions in the periodic table, formation of positive and negative ions and changes in size due to gain or loss of electrons.
Each ion has the same electron configuration (e^${–}$ – e^${–}$ repulsion), therefore more protons give a smaller radius (greater attraction) and fewer protons gives a larger radius
$\begin{array}{llllll} & Cr^{6+} & Ti^{4+} & Ca^{2+} & Cl^{-} & P^{3-} \\ \text{No. Electrons} & 18 & 18 & 18 & 18 & 18 \\ \text{No. Protons} & 24 & 22 & 20& 17& 15\end{array}$
edited Jul 23, 2014