Question

list the following ions in order of increasing ionic radius: br⁻, rb⁺, sr²⁺, se²⁻, as³⁻

Explanation:

Step1: Identify isoelectronic ions

These ions ($Br^-, Rb^+, Sr^{2+}, Se^{2-}, As^{3-}$) are isoelectronic, having the same number of electrons (36 electrons).

Step2: Understand the effect of nuclear charge

For isoelectronic species, the ion with the highest nuclear - charge (more protons) will have the smallest ionic radius as the electrons are more strongly attracted to the nucleus.
$Sr^{2+}$ has 38 protons, $Rb^+$ has 37 protons, $Br^-$ has 35 protons, $Se^{2-}$ has 34 protons and $As^{3-}$ has 33 protons.

Step3: Arrange in order of increasing ionic radius

As the nuclear charge decreases, the ionic radius increases. So the order is $Sr^{2+}

Answer:

$Sr^{2+}