Ch.7 - Periodic Properties of the Elements

Brown - Chemistry: The Central Science 15th Edition

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Brown 15th Edition

Ch.7 - Periodic Properties of the Elements

Problem 3b

Chapter 7, Problem 3b

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?

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Identify the periodic trends: Ionic size generally increases down a group and decreases across a period.

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.

Order the given ions by size: Typically, K^{+} > Cl^{-} > Se^{2-} > Mg^{2+}.

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^{+}.

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^{-}.

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Key Concepts

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

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 Se2-, 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 S2- 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, S2- is an anion with a -2 charge, which makes it larger than Cl- and Se2-. This comparison is crucial for determining the relative positions of these ions based on size.

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Textbook Question

The prefix eka- comes from the Sanskrit word for 'one.' Mendeleev used this prefix to indicate that the unknown element was one place away from the known element that followed the prefix. For example, eka-silicon, which we now call germanium, is one element below silicon. Mendeleev also predicted the existence of eka-manganese, which was not experimentally confirmed until 1937 because this element is radioactive and does not occur in nature. Based on the periodic table shown in Figure 7.1, what do we now call the element Mendeleev called eka-manganese?

Which of these spheres represents F, which represents Br, and which represents Br-?

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Textbook Question

In the following reaction

which sphere represents a metal and which represents a nonmetal?

The graph below shows the ionization energies for a particular element. In which group is the element most likely a member of? [Section 7.3]

Consider the Mg2+, Cl-, K+, and Se2- 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) S2- ions?

Verified video answer for a similar problem:

Key Concepts

Ionic Size

Trends in Ionic Radii

Comparison of Ions

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?