Metal Flame Emission

Metal Flame Emission

Flame testing is an analytical technique where a sample is placed in a flame, and the characteristic flame color is used to identify the substance. Each element has a characteristic light emission when placed into a flame, meaning each element produces a unique color. This phenomenon is used in fireworks displays, where the color of the fireworks corresponds to a specific characteristic of a metal. The emission of colored light from a burning sample is the direct result of the metallic component absorbing energy due to the excitation from the flame and emitting light as a result.

Electron Excitement

When an atomic or molecular species absorbs energy, the energy is used in four different ways. First, the energy is used in translation, causing the molecules to move faster. Second, the energy is used in vibration, which causes the distance between the species to change rapidly. Third, the energy causes rotation, which induces the rotation of atoms around the bonds in the molecule. Finally, the absorbed energy results in electron excitement, which causes electrons to move to a higher energy level from the most stable or ground state.

According to the Bohr model of hydrogen, electrons in an atom exist in discrete states, which correspond to individual shells or orbitals around the nucleus. The lowest energy state is called the ground state and is represented by the notation of n = 1. Excited energy states have higher energies and are represented by the notation of n = 2, 3, 4, etc.

For electrons to move to a higher energy state, they must absorb an amount of energy that is equal to the difference between its ground state and the higher energy state. For example, if an electron absorbs an amount of energy that is equal to the difference between the ground state and the n=3 energy level, the electron will move to the n=3 energy level. An electron can spontaneously relax back down to the ground state or any other lower energy level. When this occurs, a photon is emitted, releasing the absorbed energy.

The released energy is emitted in the form of light. The emitted light has a characteristic energy, and therefore, wavelength, that correlates to the energy levels of the atom. Visible light, which is the light humans can see with their eyes, ranges from about 400 nm to 700 nm on the electromagnetic spectrum.

Atomic absorptions and emissions are discrete wavelengths, called lines. These lines are unique characteristics of an element, like a barcode, and can be used to identify the element.

Metal Flame Emission Test

In the metal flame emission test, a metal sample is placed in a flame. The flame provides the energy to excite electrons to a higher energy level. As the electrons relax back down to the ground state, light is emitted with a specific energy relative to the energy levels of the atoms in the sample. Since different atoms have different energy levels, the energy absorbed and emitted from a sample, and thus the wavelength, is specific to the sample.

Metals have characteristic atomic emission wavelengths in the visible range that are easily determined by visual inspection. For example, lithium emits a red color, sodium emits a yellow color, potassium emits a pink-purple color, and barium emits a yellow-green color.

While atomic emissions are discrete wavelengths or lines, most samples of metals contain not only the metal, but also various metal ions, oxides, and salts. Since each atom absorbs and emits a characteristic wavelength of light, the absorbed and emitted light from the flame test contains a range of wavelengths. Thus, atomic absorption and emission spectra can be measured for the sample using a spectrophotometer.

The wavelengths and shapes of the spectra are unique for each substance. For example, the relative intensity of features in the spectrum depends on the concentration of the species. The absolute intensities are dependent on the distance from the sample to the spectrophotometer.

References

1. Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chemistry and Chemical Reactivity. Belmont, CA: Brooks/Cole, Cengage Learning.
2. Silderberg, M.S. (2009). Chemistry: The Molecular Nature of Matter and Change. Boston, MA: McGraw Hill.
3. Harris, D.C. (2015). Quantitative Chemical Analysis. New York, NY: W.H. Freeman and Company.

Flame testing is an analytical technique where a sample is applied to a flame, and the characteristic emission spectrum is used to identify different elements. When this technique is used to identify metal species, it is called metal flame emission testing. When sufficient energy is applied to some metals through a hot flame, they emit light with a characteristic wavelength or color. You can clearly see this phenomenon when watching a fireworks display.

Different metals emit different colors. Thus, we can use the color of the emitted light to determine the metal present. But before we dive into the technique further, let's take a step back. Atoms have multiple energy levels for their electrons, which are different for each element.

When an electron is at its lowest possible energy level, it is said to be at its ground state. When an atom absorbs energy, it gets excited, and electrons in the atom move from the ground state to a higher energy level. When the electrons relax back down to a lower state, or the ground state, the excess energy gained is released in the form of emitted light.

The wavelength of the emitted light depends on the energy level that the electron was excited to and the level that it relaxes back to. This emitted wavelength, or color of light, is specific to the atom present and is used to identify a metal sample in the metal flame emission test.

In the metal flame emission test, we apply a metal sample to a hot flame and observe the color of the emitted light. For example, barium will emit a yellowish-green color in the flame, while copper emits blue and potassium a pinkish-purple.

Now, a sample actually consists of the metal, metal ions, metal oxides, metal hydroxides, and metal salts. And since atoms and molecules absorb and release energy differently, the emitted light actually contains a range of wavelengths and intensities forming an emission spectrum.

A spectrophotometer is used to measure the range of wavelengths of emitted light. We can also study the absorbed wavelengths. Remember that atoms absorb energy before they release it. Absorbed light is different from emitted light.

The human eye sees emitted light, which is complementary to the absorbed light. If we look at the color wheel, we can see that complementary colors are directly across from each other. So, if the emitted light is green, that means that the sample absorbs red light.

In this lab, you'll use the metal flame emission test to identify various metals and then analyze the emitted and absorbed light using a spectrophotometer.