Element Emission Spectra
Lesson 3 of 9
Objective: SWBAT identify different elemental components of unknown emission spectra based on individual unique (and known) elemental emission spectra.
This lesson covers content that is helpful both in understanding that electrons behave as waves and how we can use emission spectra to identify elements. It is building student knowledge about electrons as we move into learning about electron configuration and how that helps predict what molecules are likely to form. It is necessary that students build their understanding of electron orbitals and valence electrons in order to address the Performance Expectation HS-PS1-2, and this lesson lays a foundation for that understanding.
Because this lesson provides students with element emission spectra which they will analyze, students are engaged in Science and Engineering Practices 4 - Analyzing and Interpreting Data. Students do not collect data, although they do have the opportunity in class to observe real element emission spectra at the beginning of the lesson so that they have some idea where these 'lines' on the emission cards come from.
Students compare the different patterns of emission spectra of elements to unknown spectra to determine what elements are present in those samples, which addresses Crosscutting Concept 1 - Patterns. Through this investigation, students continue learning how in chemistry we can use indirect observation and sampling techniques to understand what is going on at an atomic and molecular level.
While I take attendance, students do a warm-up activity in their composition Warm-Up/Reflection books. I this warm-up to probe prior learning and prepare students for the upcoming activity. (To read more about Warm Up and Reflection Books please see the attached resources.)
Today's Warm-Up: "How do we know what stars are made of?"
In this case, the warm-up is asking students to think about how we might know what something is without being able to sample it. It is also preparing them for today's activity during which students will be analyzing emission spectra of various elements.
If time permits, I walk around with a self-inking stamp to stamp the completed warm-ups indicating participation, but not necessarily accuracy. On days when there is too much business keeping, I do not stamp. Students have been told that warm-ups are occasionally immediately checked and other times not. At the end of each unit, Warm-Up/Reflection Books are collected and spot-checked.
Before class, I set up as many emission lamps as I can, depending on how many high voltage sources and different bulbs I can find. I typically can set up one at each of my lab stations, for a total of 6 - 8. I also evenly distribute all of my student spectroscopes at the lab stations so that students have easy access to them.
Emission lamp samples:
I like to use a wide range of emissions so that students can really see that some elements have very few lines and some have very many. My standard favorites are H, He, N, O, Ne, Hg, Ar, and Kr. I deliberately do NOT use any compounds (like CO) because I want to focus on individual element emissions for this introduction activity. The labels on the bulbs are rather small and difficult to see, so I do attach a post-it note to the high voltage light box with the displayed element's name.
I instruct students to break up into however many groups as I have lamps, and also how many students should be in each group. For example, in my class of 43 students, if I have 8 lamps available, I would tell my students to break into 8 groups of 5-6 students, and to choose a lab station.
Once students are settled, I explain that they will have one minute to view each spectra using the spectroscope. This is a low-stakes activity, and students are allowed a learning curve to figure out how to use the spectroscopes to view the emission lines. I will also be roaming the lab stations (as I keep time) to help those who really need it.
I turn on the lamps and turn off the lights, directing the students to look through their spectroscopes and to try to find the rainbow lines emitted from the lamps. At the end of each minute, I tell students to rotate stations. We have already established that when rotating lab stations, we move in a clockwise motion if we are looking down on the room from a top view, so students know how to move.
At the end of rotations, I tell students to leave their spectroscopes at the lab stations and to have a seat in their desks.
Transitioning to Groups
In order to transition to groups quickly, I assign groups, basically just counting out a sixth of the class and sending them to a lab station. I have 6 packs of the Element Spectroscopy cards that students will be using to investigate and compare emission spectra.
Once students are at the lab stations, I explain that in their stacks of cards, there are emission cards that look like the emission spectra that they just viewed during the station rotations. I also tell them that there are question cards.
I tell students that they need to determine the answers to the question on the cards and record their answers on a separate sheet of paper to turn in at the end of the class period. Each student is responsible to turn in their own answers, even though they are working within a small group.
Small Group Activity
Students work to match up emission spectra from the question cards with the known element emission spectra and record their answers on a sheet of paper. As students are working, I am accessible and moving from station to station to observe the student interactions and answer any questions. I am looking to see if students are having trouble with the activity and that they understand the purpose. Some questions I ask:
- What do you notice about each element's spectrum?(They each have a different set of lines.)
- How can you figure out what the unknown elements are? (We can match up the lines to see if they have the same spectrum.)
- What do we do if there are more than one element in an unknown emission spectrum?(We have to find two cards that have lines contained in the unknown spectrum.)
- How do we know what elements are in stars? (We can use a spectroscope to see what lines it has in its emission spectrum and then match it up to the element spectra that we know.)
If you do not have emission spectroscopy cards available, emission cards can be made by printing images with a color printer. One website with emission spectra available is here.
Another option is to purchase the book entitled The Elements: A Visual Exploration of Every Known Atom in the Universe.
To get a sample pack of the Element Spectroscopy card set that I use, visit here. The specific card deck that I have has 36 total cards, with 9 of them being question cards. For specific questions (and answers) please see the attached Element Emission Spectra Cards ANSWER KEY.
The included element emission spectra are:
- Mercury (through a red and blue filter, which washes out the red and blue color to make white lines)
In student's Warm-Up/Reflection Books, students should spend about 3-5 minutes writing a response to the day's reflection prompt. Prompts are designed to either help students focus on key learning goals from the day's lesson or to prompt deeper thinking. The responses also allow me to see if there are any students who are missing the mark in terms of understanding. The collection of responses in the composition books can also show a progression (or lack thereof) for individual students. (For more information about how I use Warm Up and Reflection Books, please see the attached resource.)
Today's Reflection Prompt: "Based on today's activity, how can we determine that different stars are made of different elements?"
Desired student responses should indicate that:
- Emission spectra are unique for each element (or different elements have different emission spectra).
- We can observe the emission spectra of anything burning to determine what elements are contained therein.
Here is a great sample from one of my students.