Determining Valence Electrons
Lesson 5 of 13
Objective: SWBAT recognize patterns in valence electrons in elements as organized in the periodic table.
This lesson is NGSS aligned two ways:
- HS-PS1-1 Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms. This will be the true application of this performance expectation, as we are now directly connecting patterns of valence electrons to location on the periodic table.
- Patterns Cross Cutting Concept As stated above, this will show the patterns present in the periodic table and help students understand how the behavior of atoms is related to the number of valence electrons.
I have chosen to directly instruct this material because my students are continuing to struggle with the structure of the atom. Students will have the material presented via notes, and then a chance to apply what they know in the review + practice. I want to present some certainty to build my students' confidence in their ability to work with the periodic table.
Students have experienced this concept in both the Build An Atom and Drawing the Atom simulations in Unit 1. The ExploreLearning activity on Atomic Structure was used as a reteaching opportunity for Unit 1 with all students, and introduced both valence electrons and Lewis Dot Diagrams.
The previous day the students made their Bohr-Rutherford models. On entering the classroom, I ask the students to draw an atom with four electron shells in their notebooks, and label each shell with how many electrons can fit in each shell.
Next I ask the students to get out their periodic tables. I ask students to compare the numbers in their shells with the first four periods of the periodic table and see if they recognize any similarities.
Some students will engage in this, others are content to wait back for another student to take the lead. To ensure everyone makes an attempt at the relationships, I ask the students to write down one similarity they notice between the electrons in each level and the first four periods.
Some students recall from the day before that the maximum number of electrons in the level is equal to the number of elements in the period. Others recognize that an individual shell corresponds to a particular period, such as two electrons in the first shell and two elements in period 1.
After students have had a couple minutes to think and write, we share what students noticed, drawing particular attention to the correct correlations. I then ask students to predict how many electrons would fit in the 5th level, and many students will correctly count the 5th period and come up with 18.
I ask students to keep out their periodic tables, and to turn to a clean page in their notebooks to take notes. While students are getting ready, I turn on the projector and load the Valence Electrons PowerPoint.
I tend towards simpler PowerPoints to avoid student distractions. When we do notes, I prefer them to be concrete and to the point to keep students focused. I do not dictate a notes style, some students copy the slides word for word, others hardly write anything besides examples. I do require all students to write something, and check for it in my binder checks. For more on my philosophy of note taking -- see the Notes Reflection.
We begin with how to figure out the number of valence electrons using the periodic table. Students ask why we leave out groups 3-12 and I explain that the transition metals have some complications that are beyond what we need to understand at this point. If they press on the issue, I explain that because they have space for an extra 10 electrons in the outer level, they break a lot of the rules we're about to learn, and that I don't want to confuse my students on the first go through.
Next I show how to make Lewis Dot Diagrams by lifting the screen and drawing the dots on the whiteboard around the elemental symbols. As I referenced in the introduction, this is the third time students have seen the dot diagrams, and may remember doing them previously.
The remainder of the notes are dedicated to a personalization of atoms- what they want and how they get there. We introduce the octet rule, but not yet by name. Students have a difficult time understanding how losing electrons gets to a full shell. I use the analogy of an onion with a bad spot -- when you peel off the incomplete layer with the bad spot, you have a perfectly good onion underneath. This helps students in the present, and they will get a better understanding in our Atoms and Ions lesson.
Review + Practice
I ask students to keep their notes out, and pass out the valence electron practice sheet. I give students 10 minutes to work through the sheet and then we go over it via the document camera. An example of a completed sheet is available here.
Many students do just fine on determining how many valence electrons, excepting Helium. Most students see it in group 18 and write down 8 valence electrons for it. I refer back to the periodic table and ask what period it is, and what the maximum number of electrons for the first shell is? Students realize then why it is 2, and I show the wall-mounted periodic table which represents Hydrogen and Helium above their group numbers to show that they can be exceptions to the rules.
We then go over the questions, and students readily identify that elements are using valence electrons for reactions, and that metals lose electrons and non-metals gain electrons.
Students refer to the top of the paper to answer question 4, pointing out that elements want a full valence shell, which usually means 8 valence electrons. They then explain that since group 18 already has 8, it is not reactive with other elements.
To close the lesson, I ask students to select three of the elements from the list, and write the Lewis Dot Diagrams for the elements on the back of their paper using the example from their notes.
The most common mistake at this point is pairing electrons when there are four or less. I note these mistakes for upcoming lessons when we will do more practice with the Lewis Dot Diagrams.