Ch.9 - Chemical Bonding I: The Lewis Model

Tro - Chemistry: A Molecular Approach 4th Edition

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Tro 4th Edition

Ch.9 - Chemical Bonding I: The Lewis Model

Problem 43

Chapter 9, Problem 43

What is the trend in the lattice energies of alkaline earth metal oxides? Here are the lattice energies (in kJ/mol): MgO -3795, CaO -3414, SrO -3217, BaO -3029.

Name: Chemistry: A Molecular Approach
Author: Tro
ISBN: 9780134112831

Verified step by step guidance

Identify the alkaline earth metals involved: magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba).

Recognize that lattice energy is the energy required to separate one mole of an ionic solid into its gaseous ions.

Observe the given lattice energies: MgO (-3795 kJ/mol), CaO (-3414 kJ/mol), SrO (-3217 kJ/mol), BaO (-3029 kJ/mol).

Note the trend in lattice energies: as you move down the group from MgO to BaO, the lattice energy becomes less negative, indicating a decrease in lattice energy.

Understand that this trend is due to the increasing ionic size of the metal cations as you move down the group, which results in a decrease in the electrostatic attraction between the ions.

Key Concepts

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

Lattice Energy

Lattice energy is the amount of energy released when gaseous ions combine to form an ionic solid. It is a measure of the strength of the forces between the ions in an ionic compound. Higher lattice energy indicates stronger ionic bonds, which typically results in higher melting points and greater stability of the compound.

Trends in Lattice Energy

In alkaline earth metal oxides, lattice energy generally decreases as the size of the metal cation increases. This is due to the increased distance between the ions, which weakens the electrostatic attraction. As you move down the group from magnesium to barium, the cation size increases, leading to a decrease in lattice energy.

Ionic Radius and Charge

The ionic radius is the measure of an ion's size in a crystal lattice. In alkaline earth metals, as the ionic radius increases down the group, the distance between the positively charged metal cation and the negatively charged oxide anion increases. This larger distance results in a weaker electrostatic attraction, contributing to the observed trend in decreasing lattice energies.

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