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The Sodium Doublet: Why Do We Think the Electron has Both Up and Down Spins?

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Author: Jerry Silver

The Idea

No one has ever seen an electron spin. In fact, for that matter, no one has ever even seen an electron. Yet, we know an electron behaves as if it were spinning. Some of the most revealing evidence for this comes from the light that certain atoms emit when they're excited.

If some sodium chloride is exposed to a flame, the flame takes on a characteristic yellow/orange color. This is the color observed in the common flame test used in chemistry labs to identify the presence of sodium in sodium vapor street lamps. If you look at the light coming from an excited sodium atom with a spectroscope or diffraction grating, the first thing you notice is a single orange/yellow line with a wavelength between 589 and 590 nanometers.

However, on closer inspection, you notice not one but two orange/yellow lines. The purpose of this project is to observe these two lines, known as the sodium doublet, and, more importantly, to understand why they are split.

What You Need

  • Bunsen burner or other flame
  • concentrated sodium chloride solution
  • clean nichrome wire loop (or a wooden splint)
  • diffraction grating or spectroscope
  • sodium vapor discharge tube with appropriate high-voltage power supply

Method

  1. Use one of the previous methods to produce a light source generated by excited sodium atoms.
  2. Darken the room.
  3. Observe the light using a diffraction grating or a spectroscope.
  4. Look carefully until you see a vertical yellow/orange line. Look closely until you notice this line is formed by two separate lines. See Figure 124-1.

Expected Results

The point of this project is to observe two separate yellow/orange lines that make up the sodium doublet.

Why It Works

When an electron goes from one energy level to a lower energy level, it gives off light. Each energy level can hold two electrons: one with spin up and the other with spin down. The electron with the spin up takes a slightly greater amount of energy to go from one energy level to another. As a result, the electrons with different spin conditions give off a slightly different color (wavelength) light.

Continuing the New Jersey Turnpike analogy (from Project 120), let's say you travel a certain distance going from Exit 7 to Exit 8. But things are slightly different if you get off at either an eastbound or westbound ramp at the exit. That small difference can be thought to be something like the effect caused by electron spin.

The sodium doublet. Why do we think the electron has both up and down spins

Other Things to Try

If an excited sodium atom is exposed to a very powerful magnetic field, these spectral lines split even further. This is called the Zeeman effect, which requires magnetic fields on the order of 18 Teslas. However, because this is roughly 20 times more powerful than the very strong magnetic fields used to study nuclear magnetic resonance, we won't pursue Zeeman splitting experiments in this book.

The Point

In an atom, electrons have up or down spin. When an electron goes from one energy level to another, the energy given off by each of the two spin orientations is slightly different. Observing the split in the frequency supports the concept of electron spin. Sodium gas tube

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