Common Ionization States
Lesson 4 of 9
Objective: SWBAT predict the ionization state for elements based on their tendencies to gain or lose electrons to obtain a full valence shell of electrons.
This lesson helps students understand that atoms can gain or lose electrons in order to gain stability. This understanding is necessary for students to be able to address both Performance Expectations HS-PS1-1 and HS-PS1-2. Students should understand that metals tend to lose electrons and nonmetals tend to gain in order to explain the different properties of these two types of elements, addressing HS-PS1-1. The second Performance Expectation, HS-PS1-2, requires students to be able to explain and predict outcomes of chemical reactions. Without understanding why elements bond, students cannot understand why bonds can be broken in order to form more stable subsequent bonds.
This lesson is structured to help students identify patterns in the Periodic Table so that they are not simply memorizing and are instead using the Periodic Table as a tool that they understand. This strategy is part of the Next Generation Science Standards Crosscutting Concepts, addressing Patterns. Particularly, Crosscutting Concept XC-P-HS-1 is directly addressed as students look for correlations in periodic table location to element ionization state to later explain bonding phenomena.
While I take attendance, students do a warm-up activity in their composition Warm-Up/Reflection books. I use today's warm-up to probe for students' prior knowledge about the day's upcoming lesson and to have them start thinking about what they will be learning. (To read more about Warm Up and Reflection Books, please see the attached resource.)
Today's Warm-Up: "Why don't noble gas atoms react?"
I am hopeful that some students might recall that we discussed in prior lessons that noble gases do not participate in chemical reactions because they are very stable due to their electron configurations. (See prior lessons of Introduction to Electron Orbitals and Electron Orbital Diagrams.) I anticipate that students will express a vague understanding, but will not articulate it very well.
As students complete the warm-up, I walk around and read student responses. I stamp the books of students who demonstrate any type of deeper thinking about the prompt. Answers like, "because they don't," or "they don't want to," do not get a stamp.
I ask students if anyone cares to share what their answer was, and call on one student once there are at least 5 hands in the air. After that student answers, I ask if anyone else had other ideas or if they agreed with the prior student's response. At no time during this warm-up discussion do I give students a "right" or "wrong" indication regarding their answers.
Introduction to the Activity
After our discussion about the warm-up, I tell students to take out their Periodic Tables. I ask students to think about our discussion about the warm-up prompt. I tell them to think about the electron configurations of the noble gases and I ask them what they notice. As students volunteer information sharing out, I point out (if someone has not already done so) that noble gases are at the end of the period and they have filled electron energy levels. I ask students to compare neon to sodium. Students should determine that neon has filled energy levels (both 1 and 2), but sodium does not. By adding an electron, the sodium atom enters the 3rd energy level and no longer has completely filled electron energy levels. I ask what a sodium atom would have to do in order to have the same stable electron configuration as neon. Students should answer "lose 1 electron." Our discussion continues:
- What about magnesium? It has 12 electrons. How many would it have to lose to be stable? Magnesium needs to lose 2 electrons.
- And aluminum? It has 13 electrons. How many would it have to lose? Aluminum needs to lose 3 electrons.
- Let's look at sulfur. How many electrons does it have? Sulfur has 16 electrons.
- How many electrons would it have to lose? Sulfur needs to lose 6 electrons.
- What if sulfur gained electrons instead. How many would it have to gain to be stable (like argon)? Sulfur would only need to gain 2 electrons. That seems easier than losing 6!
- So elements can either gain OR lose electrons to reach stability. Let's look for patterns on the periodic table. What will lithium do? Lithium loses 1 electron to look like helium.
- What about potassium? Potassium loses 1 electron to look like argon.
- We already saw that sodium loses one electron. Is there a pattern here? Yes! Every element in the first column loses an electron.
- What about fluorine? Fluorine gains 1 electron to look like neon.
- And chlorine? Chlorine gains 1 electron to look like argon.
- Bromine? Bromine gains 1 electron to look like krypton.
- Is there a pattern here? Yes! Every element in the second to last column gains an electron.
- So we can determine what an atom will do, either gain or lose electrons, based on its position on the periodic table. Also, metals, elements on the left side of the table, will lose electrons. If an atom loses an electron, a negative charge, what type of charge will it have? More protons than electrons means a more positive charge. Losing electrons = gaining charge.
- If nonmetals tend to gain electrons, what type of charge will they have? More electrons than protons means a more negative charge. Gaining electrons = losing charge.
I also introduce the terms "cation" for positively charged ions (because negative charge was lost) and "anion" for negatively charged ions (because negative charge was gained). I draw this picture on the whiteboard to help students remember. ("Cat"-ions are "paws"itive.)
For this activity, I allow students to work in pairs. This year, my students have had difficulty quickly choosing partners for pair activities, so I invested the time to have them create compass partners on a scheduled minimum day (where we only meet for 30 minutes) in order to save time when we do pair activities (which is quite frequently). This is the handout I used: Compass Partners Sheet. For more discussion on this idea, please see the "Compass Pairs" Reflection.
I pass out the designated sheet to students based on whether they are general level chemistry students (Ions - Anions and Cations GENERAL) or honors level chemistry (Ions - Anions and Cations HONORS.doc). I have a split level class with both honors and general level students. The honors and general level worksheets ask students to analyze the same ions, however, the honors level worksheet gives students the element symbols without ionic charge whereas the general level worksheet gives students the ionic charge on the element symbols. General level students are then asked to determine if the ion is a cation or an anion as well as how many electrons were then gained or lost. Honors level students are expected to use the periodic table to determine the appropriate predicted ionic charge for that element, then how many electrons are gained or lost.
I then ask students if they notice any elements listed in the first column (of either sheet--the question applies to both general level and honors level assignments) that are repeated. Students should point out that copper and iron are listed twice. I deliberately include transition metals with multiple possible ionization states because I want to explain that elements with d-orbital electrons can do unexpected things with their electrons in order to reach stability. I mention that they can even move electrons from the s-orbitals into d-orbitals if doing so will fill (or even half-fill) the d-orbital level. I also reassure students that I do not expect them to be able to predict transition metal ionization states, but that they do need to understand the notation that is used [i.e. Fe(II) or Fe(III)] and what it means for ionization states. I explain that since all metals tend to lose electrons, the number in the transition metal notation tells us how many electrons those atoms will lose, resulting in a positive charge of that amount.
Students work to complete their respective handouts with a due time written on the whiteboard to keep them on task. I tell students that we will be grading these as a class at the end of the period so that they understand they can not simply wait to do the work later.
Class Discussion & Grading
In my split class, my honors students typically finish an assignment like this before my general level students. However, I quickly assess the room by asking students who need more time to raise their hands, indicating how many more minutes they would like with their fingers. I choose the group who is most ready to grade first. I have those students trade papers with one person so that they are not grading their own, and I ask them to use a different color writing utensil for correcting (if the paper they are correcting is in pen, use pencil or a different color pen, etc.).
I read the correct answers (keys for each handout: Ions - Anions and Cations GENERAL KEY and Ions - Anions and Cations HONORS KEY) and display them at the same time using an LCD projector and document camera. I ask students to just write in correct answers for any incorrect answers on the paper they are correcting, and to leave the correct answers alone. This way I can quickly skim a paper to see how a student performed without having to differentiate between "correct" check marks and "incorrect" check marks. I collect corrected papers for entering grades and assessing student progress towards the learning goal.
Then, in my split level class, we grade the other level of papers using the same protocol.
My general level chemistry students performed well on this assignment. I spent most of my time in my split class helping my general level students and that might well be why they seemed to perform better overall.
This student's work was typical of most general level students and exemplifies a perfect paper.
Most students who had mistakes had minor ones, like the one shown here with an incorrect number of electrons effected listed for the aluminum.
Very few students had multiple errors, but this next paper is an example of that. This student's paper was also not "corrected" by the student grader (who was his friend) and a perfect score was put on the top by that friend. I did catch it as I glanced through the papers to check for understanding and record the grades. This is another reason to actually check up on students when we self-grade. I am less concerned that a potentially unearned perfect score might have been recorded and more concerned that this student clearly did not learn what he needed in order to move forward with constructing ionic compound formulas in following lessons.
My honors students had a wider distribution of success, perhaps because I was spending more time with my general level students. Here is a sample of a perfect paper (with an enthusiastic student grader):
This student also had a perfect paper. I like that she made a note near "cation" to describe it as positive. Her omission of the word "electron" in the second to last column near the end of the assignment was also somewhat typical as students tried to finish the assignment within the timeframe and started abbreviating their answers.
This next student clearly understood how many electrons cations lost and how many the anions gained, but failed to make the correlation to the number that belongs with the charge on the ionic symbol notation. In fact, she didn't include any numbers with the charges. This should be an easy misconception to address with this particular student since she did understand the foundation concepts.
This last sample exemplified with poorest performance, not for lack of trying on this student's part. She certainly struggled the most in trying to grasp the idea as a whole and needs more direction in even determining if elements are likely to gain or lose electrons based on their locations on the periodic table.
Today, I want to reinforce that metals tend to lose electrons resulting in a cation and nonmetals tend to gain electrons resulting in an anion. This understanding of charged ions and vocabulary (as well as tendency to gain or lose electrons based on metallic versus nonmetallic designation) is crucial to success in determining ionic molecular formulas.
Today's Reflection Prompt: "Do nonmetals gain or lose electrons? What type of charge results? Do metals gain or lose electrons? What type of charge results for them?"
Desired student answers include:
- Nonmetals tend to gain electrons which results in a negative charge.
- Metals tend to lose electrons which results in a positive charge.