In this lesson students first use the electronegativity values of elements in a compound to decide if the compound is ionic or molecular. They then look at how ionic and molecular compounds compare in terms of melting point and conductivity. These two topics are important basic information in chemistry. How compounds behave is directly related to how their atoms are bonded together. The flow of the lesson is:
This lesson aligns to the NGSS Disciplinary Core Idea of Matter and Its Interactions because students conduct an investigation about melting points and conductivity. The behaviors of these substances are quite different. Ionic compounds conduct electricity and do not melt at low temperatures, while molecular compounds act in an opposite manner. The structure of these substances at the bulk scale indicate the strength of electrical forces between their respective particles.
Due to its investigatory nature this lesson aligns to the NGSS Practice of the Scientist of Planning and Conducting an Investigation.
It aligns to the NGSS Crosscutting Concept of Cause and Effect because the learning activity shows a cause and effect relationship between the type of bonding and the physical properties each compound exhibits.
In terms of prior knowledge or skills, students should have a basic understanding of what bonding and electronegativity are.
The materials needed for this lesson include the following:
Do Now: Students enter class and are given slip of paper with the word Electrons found on it. I tell them to keep it visible and whatever they do, do not lose it!
They then work on this assignment: Read these pages in your textbook and record the answer to these questions. (A similar reading can be found at this ChemWiki website.)
I have chosen this approach because it provides students with an easy access point to the day's lesson while I focus on attendance and settling the class.
The answers they come up with can be summarized in these octet rule notes.
While they are working on this assignment I walk around like I often do, offering comments and suggestions. However, I am also stealing as many electron slips as possible.
Activator: To start class, I tell students to hold up their electron slips. I remind class that the slips are very important for today's class. Some students look stressed at this point, as I have stolen their slips. I then proceed to laugh, and show them the slips. I ask them to record in their notes the definition of electronegativity, which is the ability of an atom to steal electrons from another atom. I then refer them to a periodic table in the book that looks like this and ask them to explain what element I am like. Most go with fluorine, which is very electronegative.
I then note that if their chemistry teacher is stealing from them, then it is clear that the class has clearly gone to the dogs. I show this video that uses dogs to teach the differences between the different types of bonding. Students check their notes from the reading based on what they saw in the video.
Part 1: I show students a periodic table in their text book that lists the relative electronegativity values for the different elements, similar to one that looks like the one at this website. I explain that these are important because when atoms bond, the differences between the two atoms' electronegativities will often determine what kind of bond forms.
I explain that the following rule of thumb will apply:
Students record these notes, and I then show them an example by doing the first problem in Electronegativity and Bond Types practice problems.
I have students to this work before the lab because I want students to understand that the melting point and conductivity that they will see in the lab at the macroscale have an underlying explanation in the nanoscale arrangement of electrons.
Guided Practice: Students do a few problems and then we check in as a whole class.
Guided practice is useful here because in this way I can see if the whole class is confused by something; if most students understand the procedure I can set them free to work, and if not I can reteach the large group, which is more efficient than re-teaching the same material repeatedly to individuals. One thing this approach revealed is that I should have specified that when subtracting, subtract the smaller value from the larger. It also revealed that when the answer was zero that was confusing. I simply repeated the rules we had already discussed.
Part 2: I ask students to read the Ionic vs Molecular Properties Lab. In this lab students will see how well different compounds conduct electricity when dry and when dissolved in water, and they will see how easily the compounds melt. I cold call questions to check for understanding, and so that we can touch on the key points in the lab directions:
Once I am satisfied that students have read the lab and understand it, I release them to conduct the lab.
Part 1: Students calculate the differences in electronegativity values for the different compounds. As the student work shows, this was not a challenging exercise. Most students have similar answers; we go over these using a document projector, and then turn our attention to part 2 of the mini-lesson, as outlined above.
Part 2: After we went over the lab directions, students conducted the lab. My biggest concern is safety. I watch carefully to make sure students are being safe around the open flame, and that they are quickly removing the substance from the flame when it starts to melt, so that there is not burned sugar smoke in the room.
This video shows how students conducted the dry conductivity test. In this test, the conductivity tester does not conduct electricity because the ions are bound together and so there are no charged particles to conduct electricity. This is true for all 3 compounds.
This video shows how students conducted the dissolved conductivity test. Some students are getting a loud chirp when they tested sucrose, and so at this point in the lesson a student conducts this test in front of my watchful eye so I could observe. As it turns out, the sucrose was slightly conductive, probably due to some contamination. Some students use tap water in their tests, and some did not wipe the conductivity tester between compounds, so their sucrose conducted at a higher rate.
This video shows how students conducted the ionic vs molecular melting point test. What they find is typical of what most students find--sucrose melts and salts do not. One student claimed to melt salt, so I have him redo the test and then we discuss the importance of making sure we know what chemical we have before we conduct a test!
To wrap this lesson up I ask students to report their answers for the two tables on the handout. their answers are similar to the ones found in the Ionic vs Molecular Properties Lab answer key. I ask students whether they think the mystery compound was ionic or molecular and they seem to have consensus that it is ionic because its behavior matches that expected ionic compound more closely.
I want to make sure that students come away with a common learning experience, even if they have made some mistakes in the lab. These mistakes include contamination and not labeling their chemicals. Mistakes of this sort lead to confusion because they generate different results than what we would expect to happen when the procedure is followed correctly.
I also make a point to thank students for being safe and for following the procedure, and point out the mistakes that students made regarding contamination and confusing one chemical for another. I always try to comment on safety at the end of a lab because it is an important part of building a culture of safety in the lab.
For homework, I ask students to use their data to answer the conclusion questions,which ask them to identify bonds for the known and unknown compounds. They are also asked to evaluate the amount of energy was needed to melt each compound, and to compare their data with the rest of the class in order to evaluate its significance.