Consider the Mg²⁺, Cl^-, K⁺, and Se^2- ions. The four spheres below represent these four ions, scaled according to ionic size. (b) In terms of size, between which of the spheres would you find the (i) Ca²⁺ and (ii) S^2- ions?

Verified step by step guidance

1. <Identify the periodic trends: Ionic size generally increases down a group and decreases across a period.>
2. <Compare the given ions: Mg^{2+}, Cl^{-}, K^{+}, and Se^{2-}. Note that cations are generally smaller than their parent atoms, and anions are larger.>
3. <Order the given ions by size: Typically, K^{+} > Cl^{-} > Se^{2-} > Mg^{2+}.>
4. <Determine where Ca^{2+} fits: Ca^{2+} is larger than Mg^{2+} but smaller than K^{+}, so it would be between Mg^{2+} and K^{+}.>
5. <Determine where S^{2-} fits: S^{2-} is larger than Se^{2-} but smaller than Cl^{-}, so it would be between Se^{2-} and Cl^{-}.>

Key Concepts

Ionic Size

Ionic size refers to the radius of an ion in a crystal lattice or in solution. Cations, like Mg²⁺ and Ca²⁺, are typically smaller than their neutral atoms due to the loss of electrons, which reduces electron-electron repulsion. Conversely, anions, such as Cl^- and Se^2-, are larger than their neutral atoms because the addition of electrons increases repulsion among them.

Trends in Ionic Radii

Ionic radii exhibit periodic trends in the periodic table. Generally, ionic size increases down a group due to the addition of electron shells, while it decreases across a period from left to right due to increasing nuclear charge. Understanding these trends helps predict the relative sizes of ions, such as where Ca²⁺ and S^2- would fit among the given ions.

Comparison of Ions

When comparing ions, it is essential to consider both their charge and their position in the periodic table. For example, Ca²⁺ is a cation with a +2 charge, typically smaller than K⁺ but larger than Mg²⁺. In contrast, S^2- is an anion with a -2 charge, which makes it larger than Cl^- and Se^2-. This comparison is crucial for determining the relative positions of these ions based on size.