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MCAT Content / Electronic Structure / Absorption And Emission Line Spectra

Absorption and emission line spectra

Topic: Electronic Structure

Every element has a unique atomic absorption and emission line spectrum.

One way for atoms to gain energy is to pass an electric current through an enclosed sample of a gas at low pressure called a gas discharge tube. Since the electron energy levels are unique for each element, every gas discharge tube will glow with a distinctive color depending on the identity of the gas.

These gas discharge tubes are enclosed glass tubes filled with a gas at low pressure through which an electric current is passed. Electrons in the gaseous atoms first become excited, and then fall back to lower energy levels, emitting light of a distinctive color in the process. “Neon” signs are familiar examples of gas discharge tubes. However, only signs that glow with the red-orange color seen in the figure are filled with neon. Signs of other colors contain different gases or mixtures of gases.

Scientists studied the distinctive pink color of the gas discharge created by hydrogen gas. When a narrow beam of this light was viewed through a prism, the light was separated into four lines of particular wavelengths. An atomic emission spectrum is the pattern of lines formed when light passes through a prism to separate it into the different frequencies of light it contains.

Each of these spectral lines corresponds to a different electron transition from a higher energy state to a lower energy state. Every element has a unique atomic absorption and emission line spectrum. In the second example below, where an element has 3 excited states, it could emit photons at 6 specific wavelengths/frequencies (ΔE= hf).


Key Points

• The atomic emission spectrum is the pattern of lines formed when light passes through a prism to separate it into the different frequencies of light it contains.

• Every element has a unique atomic absorption and emission line spectrum.


Key Terms

excited-state: of an atom is a state where its potential energy is higher than the ground state.

photon: the smallest discrete amount of electromagnetic radiation. It is the basic unit of all light carrying the energy E= hf.

atomic emission spectrum: is the pattern of lines corresponds to a different electron transition from a higher energy state to a lower energy state.

gas discharge tube: is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope



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