Difference Between Atomic Absorption and Atomic Emission

The key difference between atomic absorption and atomic emission is that atomic absorption describes how atoms absorb certain wavelengths from the electromagnetic radiation whereas atomic emission describes how atoms emit certain wavelengths.

Absorption and emission of an atom help to identify atoms and provide many details about them. When absorption and emission spectra of a species are put together, they form a continuous spectrum. Therefore, atomic absorption and atomic emission are complementary to each other.

CONTENTS

1. Overview and Key Difference
2. What is Atomic Absorption 
3. What is Atomic Emission
4. Side by Side Comparison – Atomic Absorption vs Atomic Emission in Tabular Form
5. Summary

What is Atomic Absorption?

A coloured compound is visible to our eyes in that particular colour because it absorbs light from the visible range. In fact, it absorbs the complementary colour of the colour we see. For example, we see an object as green because it absorbs purple light from the visible range. Thus, purple is the complementary colour of green. Likewise, atoms or molecules also absorb certain wavelengths from the electromagnetic radiation (these wavelengths are not necessarily in the visible range). When a beam of electromagnetic radiation passes through a sample containing gaseous atoms, the atoms absorb only some wavelengths. The absorbed energy helps to excite ground electrons to upper levels in the atom. We call this electronic transition. The energy difference between the two levels is supplied by the photons in the electromagnetic radiation.

Since this energy difference is discrete and constant, the same kind of atoms will always absorb the same wavelengths from the given radiation. An absorption spectrum is a plot drawn between absorbance and wavelength. Sometimes instead of wavelength, frequency or wave number can also be used in the x-axis. Log absorption value or the transmission value is also used for the y-axis in some occasions.

After light passes through an atomic sample, if we record it, we can call it an atomic spectrum. It shows the characteristic of a type of atom. Therefore, we can use it in identifying or confirming the identity of a particular species. And, this kind of spectrum will have a number of very narrow absorption lines.

What is Atomic Emission?

Atomic emission is the emission of electromagnetic radiation from atoms. Atoms can be excited to a higher energy level if the required amount of energy is provided externally. The lifetime of an excited state is generally short. Therefore, these excited species have to release the absorbed energy and come back to the ground state. We call this relaxation.

The release of energy may take place as electromagnetic radiation, heat or as both types. The plot of released energy versus wavelength gives an emission spectrum.

Each element has a unique emission spectrum as they have a unique absorption spectrum. Therefore, we can characterize radiation from a source by emission. Line spectra occur when the radiating species are individual atomic particles that are well separated in a gas.

What is the Difference Between Atomic Absorption and Atomic Emission?

Atomic absorption is the absorption of electromagnetic radiation by atoms while atomic emission is the emission of electromagnetic radiation from atoms. So, the key difference between atomic absorption and atomic emission is that atomic absorption describes how atoms absorb certain wavelengths from the electromagnetic radiation whereas atomic emission describes how atoms emit certain wavelengths.

Furthermore, atomic absorption requires a source that can emit radiation while atomic emission occurs even in the absence of a source that emits radiation. Moreover, by absorbing, the electrons in an atom are excited to a higher energy level. By emission, excited electrons are coming back to a lower level. Therefore, this is another significant difference between atomic absorption and atomic emission.

Summary – Atomic Absorption vs Atomic Emission

Atomic absorption and atomic emission are two opposite phenomena that take place simultaneously. The key difference between atomic absorption and atomic emission is that atomic absorption describes how atoms absorb certain wavelengths from the electromagnetic radiation whereas atomic emission describes how atoms emit certain wavelengths.

Reference:

1. Lindon, John C., et al. Encyclopedia of Spectroscopy and Spectrometry. Elsevier, 2017.
2. “Absorption (Electromagnetic Radiation).” Wikipedia, Wikimedia Foundation, 3 May 2019, Available here.

Image Courtesy:

1. “Two level system” By Philipposc – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “11 Oxygen Spectrum” By Image Editor (CC BY 2.0) via Flickr