Difference Between Absorption and Emission Spectra

Main Difference – Absorption vs Emission Spectra

The structure of an atom includes a central core called a nucleus and a cloud of electrons around the nucleus. According to the modern atomic theory, these electrons are positioned in specific energy levels called shells or orbitals where their energies are quantized. The shell which is the nearest to the nucleus is known to have the lowest energy. When energy is given to an atom externally, it causes the electrons to jump from one shell to another. These movements can be used to obtain absorption and emission spectra. Both absorption and emission spectra are line spectra. The main difference between absorption and emission spectra is that absorption spectra show black colored gaps/lines whereas emission spectra show different colored lines in the spectra.  

Key Areas Covered

1. What are Absorption Spectra
      – Definition, Characteristics
2. What are Emission Spectra
      – Definition, Characteristics
3. What is the difference between Absorption and Emission Spectra
     – Comparison of Key Differences

Key Terms: Atom, Absorption Spectra, Emission Spectra, Orbital, Photon, Shell

What are Absorption Spectra

An absorption spectrum can be defined as a spectrum obtained by transmitting electromagnetic radiation through a substance. The characteristic feature of an absorption spectra is that it shows dark lines on the spectrum.

Absorption spectrum is a result of absorbing photons by the atoms present in the substance. When a substance is exposed to an electromagnetic radiation source such as white light, it can obtain the absorption spectra. If the energy of the photon is the same as the energy between two energy levels, then the photon’s energy is absorbed by the electron in the lower energy level. This absorption causes the energy of that particular electron to be increased. Then the energy of that electron is high. Thus, it jumps to the higher energy level. But if the energy of the photon is not equal to the energy difference between two energy levels, the photon is not going to be absorbed.

Then the transmission of the radiation through the substance gives colored bands that correspond to the photons that were not absorbed; dark lines indicate the photons which were absorbed. The energy of a photon is given as;

E = hc / λ

Where, E – energy of the photon (Jmol^-1)            c – Speed of radiation (ms^-1)

          h – Plank’s constant (Js)                             λ – Wavelength (m)      

Therefore, the energy is inversely proportional to the wavelength of the electromagnetic radiation. Since the continuous spectrum of the light source is given as the wavelength range of electromagnetic radiation, the missing wavelengths can be found. The energy levels and their locations in an atom can also be determined from this. This indicates that an absorption spectrum is specific to a particular atom.

Figure 1: Absorption Spectrum of few elements

What are Emission Spectra

Emission spectrum can be defined as a spectrum of the electromagnetic radiation emitted by a substance. An atom emits electromagnetic radiation when it comes to a stable state from an excited state. Excited atoms have a higher energy. In order to become stable, atoms should come to a lower energy state. Their energy is released as photons. This collection of photons together makes a spectrum known as the emission spectrum.

An emission spectrum shows colored lines or bands in the spectrum because the released photons have a specific wavelength corresponding to that particular wavelength of the continuous spectrum. Therefore, the color of that wavelength in the continuous spectrum is shown by the emission spectrum.

The emission spectrum is unique to a substance. This is because the emission spectrum is exactly the inverse of the absorption spectrum.

Figure 2: Emission Spectrum of Helium

Difference Between Absorption and Emission Spectra

Definition

Absorption Spectra: An absorption spectrum can be defined as a spectrum obtained by transmitting electromagnetic radiation through a substance.

Emission Spectra: Emission spectrum can be defined as a spectrum of the electromagnetic radiation emitted by a substance.

Energy Consumption

Absorption Spectra: An absorption spectrum is produced when atoms absorb energy.

Emission Spectra: An emission spectrum is produced when atoms release energy.

Appearance

Absorption Spectra: Absorption spectra show dark lines or gaps.

Emission Spectra: Emission spectra show colored lines.

Energy of Atom

Absorption Spectra: An atom obtains a higher energy level when an absorption spectrum is given by that atom.

Emission Spectra: An emission spectrum is given when an excited atom obtains a lower energy level.

Wavelength

Absorption Spectra: Absorption spectra account for wavelengths absorbed by a substance.

Emission Spectra: Emission spectra account for the wavelengths emitted by a substance.

Summary

Line spectra are very useful in determining an unknown substance because these spectra are unique to a particular substance. The major types of spectra are continuous spectra, absorption spectra, and emission spectra. The main difference between absorption and emission spectra is that absorption spectra show black colored gaps/lines whereas emission spectra show different colored lines.

References:

1.”Absorption and Emission Spectra.” Department of Astronomy and Astrophysics. N.p., n.d. Web. Available here. 19 June 2017. 
2.”Emission and absorption spectra.” Everything Maths and Science. N.p., n.d. Web. Available here. 19 June 2017. 

Image Courtesy:

1. “Absorption spectrum of few elements” By Almuazi – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Visible spectrum of helium” By Jan Homann – Own work (CC BY-SA 3.0) via Commons Wikimedia