The Secret Ingredient for Teaching Ionization Energy Without Guesswork
For some students, picking out periodic trends on the table feels more like a witch stirring a cauldron of all the elements, reaching in and grabbing a random one out. Double, double toil and trouble; Fire burn and cauldron bubble. Oh, and boron has lower first ionization energy than beryllium.
First ionization energy (IE1) can be a tricky concept for some grade-level chemistry students to understand. There is the general trend, but also the exceptions to the trend. Predicting these relative values can leave a student feeling like they are just reaching in the pot and grabbing a random answer.
And guessing isn’t learning.
My antidote to this problem is combining three understandable traits that lead to the IE1 trend; Zeff, radius, and stability. But first, attraction.
Teach Coulomb’s Law first.
The force of attraction found between charged particles in the atom gives students an understandable anchor to base many trends on. The first thing I teach them in this unit is the format of Coulomb’s Law. We never actually do the math directly, but it is easy to see trends. As charge (q1) goes up, attraction goes up. As radius goes up, attraction goes down. This is a great piece of leverage for the unit.
Effective nuclear charge (Zeff) is directly proportional to ionization energy.
Zeff is the first of the three components to predict ionization energy. I substitute Zeff for q1 on the top of Coulomb’s law. That allows the kids to see that as Zeff increases so does first ionization energy. This sets the table for them to understand that each new energy level will have a generally increasing first ionization energy.
Radius is inversely proportional to first ionization energy.
After Zeff the next trend the students learn is radius. More effective nuclear charge means smaller radius. More energy levels mean higher radius. Then as we approach first ionization energy, the students can see the general trend that moving down a period leads to lower force of attraction due to increasing radius. Also, as we move across the period, IE1 will generally increase as radius decreases.
Teaching Zeff and radius combined with Coulomb’s Law has set the table for adding the exceptions.
Stability wins.
As soon as students can predict the general trend of first ionization energy, I have them analyze a table of data. They have to explain each change in IE1. Which they can do just fine. Until boron.
Boron has a smaller radius and a higher Zeff but a lower first ionization energy. Something else must be at play. Enter stability.
My students have been trained since the beginning of the year on what we call the Jesus answer. I teach at a Christian school, and the kids know that the answer in Sunday school is almost always Jesus. So, the answer in chemistry is almost always “to become more stable”. Why does this or that happen? To become more stable. It’s practically our mantra.
When I introduce the idea of stability affecting the first ionization energy trend, it isn’t that big of a reach to tell them that stability wins.
The ultimate win is full s and p subshells at the noble gases. We call this very stable. Then from the analysis of the table, the students recognize that there are two more stable spots on the table; full s subshell and half-full p subshell. We call these “kinda stable”.
Use Zeff, radius, and stability to predict the trend.
I give my students lots of practice on choosing between two elements to identify which has the lowest first ionization energy.
Here is a simple worksheet that we used for review yesterday.
They simply apply the three traits and estimate the level of attraction for the valence electrons. When in doubt, stability wins.
This method of teaching the trend allows them to step away from the randomness of the cauldron of elements and make informed decisions. Much less boiling, toiling and troubling.
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