According to the Bohr model of the atom, electrons are confined to orbits with fixed distances around the nucleus that correspond to specific levels of energy. Higher energy levels are located farther from the nucleus. When atoms are bombarded with energy, each electron can only absorb the precise quantum of energy needed to move from its ground state to a higher energy level (excited state). When electrons fall back to the ground state, this quantum of energy is released as a photon of light. Each element has a unique set of electrons, and therefore, will emit a unique spectrum of photons. These emission spectra can be used like a fingerprint to identify an element! Here’s an animated video that I made of the phenomenon.
What makes this argument-driven inquiry lab unique and challenging for most students is the authentic nature of the inquiry. Students were given a question and shown the proper use of laboratory tools, but they were not given a procedure, nor a data table. By considering what data they would need for their argument, and how they would produce and record their data, they gained a deeper understanding of the subject matter, as well as their skill in the science and engineering practices.
After their plans were approved, students began to energize atoms in a bunsen burner flame and analyze the light with a spectroscope. The spectroscope separates the photons by diffraction and allows each wavelength to be measured for comparison. In the photo above you can see by the scale that the bright yellow spectral line has a wavelength of 604 nanometers. Students collected spectral emissions from a variety of known substances, and then unknown substances. Most were able to use a cell phone to capture images through the spectroscopes. They collaborated by sharing the data via uploads to Google Drive. In their notebooks their lab reports will include a scannable QR code that will open the link to their shared photos. Here is a QR code example that will direct you to the section of my website where resources for this lab are located. There are many websites for generating QR codes. This one was done at QR Stuff
After gathering their evidence, each lab group worked together on a whiteboard to create and share an argument that consists of the following:
- Their claim – They will answer the question, What are the identities of the unknown substances?
- Their evidence – They will present their photographs of the emission spectra and measurements of prominent lines.
- Their Reasoning – They will justify their claim by explaining the significance of their evidence by relating it to the principles and theory they have studied.
After sharing and revising their arguments with other lab groups, each student typed an individual report. Their reports were then edited after a double-blind group peer review. They submitted their edited final draft through Turnitin.
Here are a few questions for further consideration. Please feel free to comment (extra credit?).
- In this lab we studied emission spectra. So what are the differences and similarities between an emission spectrum and an absorption spectrum?
- We use emission and absorption spectra to study nature at the atomic scale (which is unimaginably small), so how do astronomers use the same phenomena to study the vast universe?
- What is the relationship between the wavelengths of the spectral lines and their frequency? Can you calculate the frequency for one of the lines in the helium emission spectrum above?
- What is the relationship between the frequency of the photons and their energy? Can you calculate the energy for one of the lines in the helium emission spectrum above?
- What’s up with Planck’s Constant? What does it reveal about the quantum nature of the atom, and the wave-particle duality of light?