Rank the following in order of increasing ionic radius: Br-, Kr, Rb+

Question

BlackTom AI · Accepted Answer

Start by identifying the species and what 'increasing ionic radius' means in this context. We are ordering by size from smallest to largest.
Option by option analysis:
- Option: Rb+<Br-<Kr
This would place Rb+ as the smallest and Kr as the largest. In reality, Kr (a noble gas with no additional charge and a relatively compact van der Waals radius) is smaller than many ionic species, and Br- carries an extra electron which tends to increase its radius compared to neutral or positively charged species. This ordering contradicts known radii relationships, so this option is incorrect.
- Option: Br-<Kr<Rb+
Here Br- is placed smallest and Rb+ largest. However, Br- is a fairly large anion due to the extra electron repulsion among the many electrons, generally larger than a cation of similar electron count. Also, Rb+ is typically smaller than Br- because cations (especially heavier alkali metal cations) lose electrons and their effective nuclear charge pulls electrons closer, reducing size. This option misorders both Br- relative to Kr and Kr relative to Rb+, so it’s incorrect.
- Option: Kr<Rb+<Br-
This option places Kr as the smallest, followed by Rb+, and Br- as the largest. The key reasoning: Kr has a closed shell with relatively low size; Rb+ has fewer electrons than neutral Rb and a relatively strong nuclear pull, making it smaller than Br-; Br- is larger due to extra electron repulsion and its anionic state. This sequence aligns with standard trends for these species, so this option is plausibly correct.
- Option: Rb+<Kr<Br-
This would imply Kr is larger than Rb+. But since Rb+ has more protons pulling on the same number of electrons compared to Kr (Rb+ has Z=37, Kr has Z=36, both with 36 electrons), the additional proton in Rb+ makes its radius smaller, not larger, than Kr. Therefore this ordering is inconsistent with atomic/ionic size trends and is incorrect.
- Option: Kr<Br-<Rb+
This places Br- between Kr and Rb+. However, Br- generally has a larger radius than Kr and often larger than many cations of comparable electron count; putting Rb+ as the largest here is not consistent with the typical size hierarchy (Rb+ is usually smaller than Br-). This option is incorrect.
- Option: Br-<Rb+<Kr
This order claims Br- is smallest, then Rb+, then Kr largest. This contradicts the well-established trend that Kr (a neutral noble gas with a compact electron cloud) is smaller than Br- and that Br- is typically larger than many cations like Rb+. The progression is inverted and thus incorrect.

Bringing these assessments together, the arrangement Kr<Rb+<Br- matches the known sizes: krypton’s atomic-like radius is small, the rubidium cation is moderately small due to electron loss and higher effective nuclear charge, and bromide’s extra electron makes it the largest among the three. Consequently, the only option consistent with ionic and atomic size trends is the third choice.

Rank the following in order of increasing ionic radius: Br-, Kr, Rb+

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