Accepting the exceptions

This exercise is a part of Educator Guide: Chemical Bond Acts Like a Mash-Up / View Guide

Directions for teachers:

Use the online Science News article This weird chemical bond acts like a mash-up of hydrogen and covalent bonds” and the prompts below to have students explore classical definitions of chemical bonding and how those definitions should be revised based on new research. A version of the story, “Chemical bond acts like a mash-up,” appears in the January 30, 2021 issue of Science News. As a final exercise, have students discuss how to best learn general chemistry concepts and their exceptions.

Want to make it a virtual lesson? Post the online Science News article“This weird chemical bond acts like a mash-up of hydrogen and covalent bonds,” to your learning management system. Pair up students and allow them to connect via virtual breakout rooms in a video conference, over the phone, in a shared document or using another chat system. Have each pair post its answers to the third set of questions, or conduct a class discussion to allow groups to share out.

Classical definitions

Discuss the following questions with a partner before reading the Science News article. Reference an outside resource if needed, but try to determine examples on your own.

1. What does electrostatic attraction mean? How does this concept apply to atoms and chemical bonding?

Electrostatic attraction is the electric force between two oppositely charged bodies. The force of attraction between positively charged protons and negatively charged electrons within an atom, and the attractive forces between partially or fully charged molecules in a substance, impacts the chemical reactivity of the substance.

2. What is the difference between intermolecular and intramolecular attraction forces? Explain and give an example of each based on your understanding of the concepts.  

Intermolecular attraction forces are attractive forces between molecules. Intramolecular attraction forces are the attractive forces between atoms within a molecule. Generally, hydrogen bonds and dipole-diploe interactions are considered intermolecular attraction forces. Covalent bonds, both polar and nonpolar, are typically defined as intramolecular attractions. Ionic bonds are also generally thought of intramolecular forces. They are forces of attraction between charged atoms and/or molecules. Water, or H2O, can be used as an example of both intra-and intermolecular attraction forces. Hydrogen bonds occur between molecules of H2O, but within one H2O molecule, each hydrogen atom forms a polar covalent bond with the lone oxygen atom in the molecule. 

3. What type of attraction force is generally thought of as a “true chemical bond?” What does this tell you about the general difference in attractive strength of intermolecular versus intramolecular attraction forces?

Intramolecular attraction forces are generally viewed as true chemical bonds. These forces are very strong, meaning a lot of energy is generally required to separate atoms within molecules. Intermolecular attraction forces are generally less strong, meaning it doesn’t take much energy to separate molecules. 

4. Why do we attempt to classify types of chemical bonds and/or attractive forces within a substance or mixture?

To predict the physical and chemical properties of a substance, such as the ability to transport hydrogen ions, as mentioned in the Science News article.  

Modifying definitions

Read the online Science News article “This weird chemical bond acts like a mash-up of hydrogen and covalent bonds,” and answer the following questions individually, before discussing them with a partner.

1. What is a hydrogen-mediated chemical bond and why is it unique?

A hydrogen-mediated chemical bond is a hybrid of a hydrogen bond and a covalent bond. It is stronger than a typical hydrogen bond and involves electron sharing, which is typically characteristic of covalent bonding.

2. Given the information in the article, what chemistry terms need to be redefined? Why?

Molecules and chemical bonds. The newly discovered hydrogen-mediated chemical bond does not fit into the conventional bond categories. It also redefines the conventional knowledge about what constitutes a molecule.

Exceptions to the rules

Discuss the following questions with a classmate. Write down your thoughts and be prepared to share your answers with the class.
1. Give an example of an exception to a generalized chemistry concept that you learned about this year and explain why the exception exists. For example, were there exceptions to trends on the periodic table, classifications of properties of certain types of substances (acids/bases, conductors/insulators, etc.), or other theories (kinetic molecular theory, etc.)? 

When learning the trends on the periodic table, there are exceptions to trends in ionization energy, or the energy required to remove the outermost electron from an atom. Generally, the ionization energy of elements increases as you move from left to right across a row of the periodic table. However, oxygen has a lower first ionization energy than nitrogen. This exception is explained by the fact that oxygen atoms have a paired electron in its outer most energy level which, based on the repulsive forces of that pairing, makes the outermost electron less stable and more easily removed from oxygen.

2. Based on the new research described in the Science News article, write an exception to the generalized concept of chemical bonding.

Generally speaking, intermolecular attraction forces are weaker than intramolecular attractive forces (also called true chemical bonds). However, the hydrogen-mediated bond is an exception to this rule. This bond is a hybrid of intramolecular and intermolecular attractive forces — it acts as both a covalent bond and a hydrogen bond.

3. Think about how you would prefer to learn information. Would you rather learn a set of generalized concepts then think through and explain the exceptions to those concepts? Or would you rather assume that there are no generalized concepts and evaluate each phenomenon on a case-by-case basis? Before answering, discuss the benefits and drawbacks of each option, and why you think the way that you do.

Student answers will vary, but will likely include something about the simplicity and the ease of using generalizations and categories to understand many or most specific examples. Treating each example as an individual case to memorize or look up would require much more time and effort. The downside of making generalizations is that it often causes us to overlook or minimize the exceptions, because they aren’t the norm. Students should also give their opinion about the way that they would prefer to learn the material. They may also say that they would like to go with the generalizations, even though remembering the exceptions to the rule can be a pain.