Atomic Emission Tubes and Spectroscopy (James Rice)

Author

James Rice - Northridge Academy High School

Principles

1. The Electromagnetic Spectrum

• "The entire range of wavelengths or frequencies of electromagnetic radiation extending from gamma rays to the longest radio waves and including visible light." (Google Definition)

2. Visible Light Spectrum and Atomic Emission

• The emission spectrum of a chemical element is the spectrum of electromagnetic radiation emitted by the element's atoms.

• Each element's emission spectrum is unique. Therefore, spectroscopy can be used to identify the elements in matter of unknown composition. This is one of the ways we figure out the composition of stars.

3. Atomic Theory and Bohr's Model of the Atom

• Bohr's model of the energy levels of electrons provides a model for explaining the emission phenomenon with respect to the precise wavelengths of light emitted by each element.

Standards

Science and Engineering Practices:

Developing and Using Models

Modeling in 9–12 builds on K–8 and progresses to using, synthesizing, and developing models to predict and show relationships among variables between systems and their components in the natural and designed worlds.

• Use a model to predict the relationships between systems or between components of a system.

Disciplinary Core Ideas:

PS1.A: Structure and Properties of Matter

• Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons.

• The periodic table orders elements horizontally by the number of protons in the atom’s nucleus and places those with similar chemical properties in columns. The repeating patterns of this table reflect patterns of outer electron states.

Crosscutting Concepts:

Patterns

• Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena.

Materials needed

• Handheld diffractions gratings or spectroscopes are particularly helpful for allowing students to visualize the unique spectra that can be produced by atomic excitation of the various element samples.

• Smartphones to record still and video images of spectra.

Procedure

1. Plug in the power source with the power switch set to "off".

2. Insert the emission tube into the holder (gently).

3. Switch the power source to "on".

4. Keep hands and objects away from the current-carrying contacts on either end of the spectrum tube. Do not run continuously for more than 30s at a time.

5. Switch power source to "off".

If allowed to run for more than 30s at a time, emission tubes can become dangerously hot and can potentially fuse, destroying the tube and also potentially damaging the power source.

Explanation

According to the Bohr model of the atom, the electrons in the atom are all arranged at various energy levels. The arrangement of those electrons in which all of them are in their lowest possible energy state, as a group, is called the ground state electron configuration for that element. However, any of the individual electrons can absorb energy (either from a photon, collision with high energy electrons, atomic collision) and move to a higher energy level, creating what is called an excited state. There is only one possible ground state for an atom, but there are many possible excited states.

However, although meta-stable, each excited state will not last indefinitely, and eventually an excited electron will lose return to a lower energy level and release some or all of the energy it had previously absorbed. If it returns to the ground state after having absorbed electromagnetic energy from a photon, it will release a photon with the same electromagnetic energy, i.e. light of the same wavelength. However, if that electron drops from the excited state to a state that is lower than its original excited state, but not all the way back to the ground state, it may release less than the amount of energy it originally absorbed, and thus it can radiate out electromagnetic radiation of a different frequency than any that was absorbed.

The combination of all the various possible wavelengths of light that can be emitted from different de-excitation transitions within atoms of a given element is what makes up the emission spectrum for that element, as in the example below. For the example below, the n=6 to n=2 transition releases electromagnetic radiation in the near-UV range. The next transition, n=5 to n=2, releases violet, n=4 to n=2 releases a blue-green, and n=3 to n=2 releases red. It is worth noting that all of these transitions are not returning the atom all the way to the ground state, but because returning to n=1 releases significantly more energy than is allowed for EM radiation in the 400 nm - to 700 nm range, those transitions are invisible to the human eye, starting in the UV and moving to even lower wavelengths.

Questions

• How can an atomic emission tube demonstrate the quantum nature of the atom?

  • The reason that emission tubes create emissions of individual wavelengths, rather than a continuous spectrum, like that from the sun, is because that the gas is not heated to a high enough temperature to give off light due to the blackbody radiation effect. Instead, electrons inside the atoms have been energized to an excited state and moved to higher energy levels within those atoms, only to revert to their ground states eventually. In transitioning back to a ground state, each atom gives off electromagnetic radiation (aka light) with an energy equal to the energy drop the electron experiences. Each energy of EM radiation corresponds to a specific wavelength of light. So, each of those wavelengths of light is evidence that there are specific, quantized energy levels inside the atom, which points to the quantum nature of atoms.

• Why is it that each emission spectrum is unique to its element?

  • As noted in response to the first question, the quantized nature of electron energy levels allows for electron de-excitation transitions that emit very specific energies of light. Those specific wavelengths, in combination, make up an emission spectrum. However, the exact energies of the energy levels within the atom are determined by a number of factors unique to each element. The primary factor is the nuclear charge of the element. As more protons are added to the nucleus of an atom, the electric potential field within the atom changes. That potential field is what defines the energy levels of the atom. So, because the number of protons in the atom also defines which element an atom is, that means that every element has its own unique electric potential field, and thus its own unique set of energy levels among which electrons can transition. So, each atom has its own unique set of possible transitions, and therefore unique set of wavelengths, or spectrum.

• What do you think would happen to the emission spectrum if two different elements were mixed in the same tube?

  • Because every element has a unique spectrum, and the emission spectra are determined by factors within each atom that are not easily influenced by conditions outside the atom, then the individual emission spectrum from each element would not be affected by the presence of other elements. In other words, the two spectra would simply both exist at the same time. This would mean that any spectral lines shared by elements would appear brighter than they would otherwise as they would be overlapping light from two different sources, but the remainder of the spectrum of each of the elements would remain undisturbed. It actually by using this underlying assumption that the element helium was discovered.

Everyday examples of the principles illustrated

• Fluorescent tubes and other light-emitting tubes produce emission spectra of roughly individual wavelengths, but these spectra, as they are often used for illumination purposes, don't always serve their purpose well because they don't include the full visible spectrum of light the way that sunlight does. This is why, when trying on clothes indoors under fluorescent lights, colors will look one way, but appear differently, some times to dramatic effect, outdoors. Typical fluorescent tubes are generally filled with a mercury gas whose spectral peaks include red, green, and blue, such that the chromophores in our eyes are stimulated and we perceive "white" light, but many wavelengths were are sensitive to are missing. A similar effect occurs when using sodium vapor lamps, particularly in municipal outdoor lighting. Although sodium vapor lamps are now becoming outmoded, they are still used extensively throughout the world.

• Light Amplification by Stimulated Emission Resonance (LASER) was once an acronym, but has become so ubiquitous as to be completely un-noticed in the modern day. Before

• solid-state materials, they alsoworkby the same excited-state emission process that gas discharge tubes do. Even more so than lasers, LEDs are so thoroughly ubiquitous in the modern world as to be almostinvisible. Nearly every monochrome (single-color) indicator light on any piece ofelectronic equipment is lit by an LED, as well as the clock faces of alarm clocks that can be read in the dark. Of course, in the case of white LEDs, this would seem a bit mystifying since there is no single wavelength of light that creates what humans perceive as "white". Of course, "white" LEDs are a relatively recent invention. The original LEDs were red, and later consumer models were green, but it took considerably more effort to develop the kinds of blue LEDs that are now commonly taken for granted. The invention of the blue LED signalled a transformation in the possibilities for creation of short-wavelength solid-state lasers, and the development of the BluRay player is one of the most notable developments to use that fundamental technology. Also, because of their relatively high energy efficiency, low thermal output, and long-lasting nature, RGB screens made of millions of LED pixels have become the standard in large-format digital graphic display boards around the world. If you have ever been to a sports stadium or driven past a motion graphic billboard, then you have witnessed atomic emissions in action.

Photos

Movies

References

• Wikipedia: Gas Discharge Lamps

• Wikipedia: Bohr Model of the Atom

• Wikipedia: Emission Spectrum

• Wikipedia: Electromagnetic Spectrum