Order the following ions from smallest to largest: Sr2+, Se2-, Br-, Rb+.

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insert step 1> Identify the ions and their respective charges: Sr^{2+}, Se^{2-}, Br^{-}, Rb^{+}.

insert step 2> Recognize that ions with the same number of electrons are isoelectronic.

insert step 3> Determine the electron configuration for each ion: Sr^{2+} (36 electrons), Se^{2-} (36 electrons), Br^{-} (36 electrons), Rb^{+} (36 electrons).

insert step 4> Understand that for isoelectronic ions, the size decreases with increasing nuclear charge (more protons).

insert step 5> Order the ions based on increasing nuclear charge: Se^{2-} < Br^{-} < Rb^{+} < Sr^{2+}.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ionic Radius

Ionic radius refers to the size of an ion in a crystal lattice. Cations (positively charged ions) are generally smaller than their neutral atoms due to the loss of electrons, which reduces electron-electron repulsion and allows the remaining electrons to be pulled closer to the nucleus. Conversely, anions (negatively charged ions) are larger than their neutral atoms because the addition of electrons increases repulsion among them, causing the electron cloud to expand.

Charge and Size Relationship

The charge of an ion significantly influences its size. Cations, such as Sr2+, have a higher positive charge, leading to a smaller ionic radius compared to their neutral counterparts. Anions, like Se2- and Br-, have negative charges that increase their size due to added electrons. Thus, the greater the charge (in absolute value), the smaller the cation and the larger the anion, affecting their relative sizes.

Periodic Trends

Periodic trends in the periodic table help predict the relative sizes of ions. As you move down a group, ionic size increases due to the addition of electron shells. Across a period, ionic size generally decreases for cations and increases for anions. Understanding these trends allows for the comparison of different ions, such as Sr2+, Se2-, Br-, and Rb+, in terms of their sizes based on their positions in the periodic table.