In complete sentences, explain: (a) why Se^2- and Br^- are about the same size; (b) why Br^- is

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In complete sentences, explain: (a) why Se^2- and Br^- are...

In complete sentences, explain: (a) why $\mathrm{Se}^{2-}$ and $\mathrm{Br}^{-}$ are about the same size; (b) why $\mathrm{Br}^{-}$ is slightly smaller than $\mathrm{Se}^{2-} ;$ and $(\mathrm{c})$ which singly charged cation you would expect to be approximately the same size as $\mathrm{Se}^{2-}$ and $\mathrm{Br}^{-}$ and why.

In complete sentences, explain: (a) why $\mathrm{Se}^{2-}$ and $\mathrm{Br}^{-}$ are about
the same size; (b) why $\mathrm{Br}^{-}$ is slightly smaller than $\mathrm{Se}^{2-} ;$ and $(\mathrm{c})$ which singly charged cation you would expect to be approximately the same size as $\mathrm{Se}^{2-}$ and $\mathrm{Br}^{-}$ and why.

Key Concepts

Isoelectronic Species

Isoelectronic species are atoms or ions that have the same number of electrons and therefore the same electron configuration. This concept is fundamental when comparing the sizes of different ions that, despite having different numbers of protons, share the same electron shell arrangement, which provides a common basis for analyzing differences in ionic radii.

Effective Nuclear Charge

Effective nuclear charge refers to the net positive charge experienced by electrons in an atom or ion after accounting for the shielding effect of the inner electrons. In an isoelectronic series, species with a higher effective nuclear charge pull their electron cloud more tightly, resulting in a smaller ionic radius. This concept explains why two isoelectronic ions can have slightly different sizes based on the number of protons in their nuclei.

Trends in Ionic Radii

Ionic radii trends consider both the total number of electrons and the strength of the nuclear charge acting on them. Even among isoelectronic species, differences in the nuclear charge cause variations in size. With a higher nuclear charge, the electrons are drawn closer to the nucleus, leading to a smaller radius; conversely, a lower nuclear charge will have a less contracted electron cloud, resulting in a larger ionic radius.

Isoelectronic Cation Identification

When identifying a cation that is isoelectronic with given anions, the key factor is ensuring the cation has the same electron count. By selecting an element whose atomic number, when reduced by one due to ion formation, equals the electron count of the isoelectronic anions, one can find a cation with a similar electron configuration. This method reliably predicts that the cation will have a comparable ionic radius to the isoelectronic species due to similar electron cloud structures.

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