Emission and Absorption Spectra: Explanation and Sample Questions

The study of atoms or atomic ions through their interaction with Electro-magnetic radiation is called Atomic Spectra. An atom is the smallest unit of an ordinary matter that uniquely defines the structure of a chemical element. During the transition between different levels of energy within an atom, a spectrum of electromagnetic radiation is emitted or absorbed by the Electron present in the atom in the form of Light. The difference in the speed of light in various mediums causes bending when light travels from one medium to another. Let us understand the meaning of Emission and Absorption Spectrum 

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Emission Spectrum

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Emission Spectrum is defined as the spectrum of frequencies of Electromagnetic Radiation that is emitted while making a transition from a higher energy state to lower energy by an atom or molecule. The collection of different radiated wavelengths help to build an Emission Spectrum. These emission spectra are widely used for the chemical analysis of a chemical substance.

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Highlights on Emission Spectrum

1. During the interaction of electromagnetic radiation with molecules of matter or atoms, the energy helps the electron and push it to a higher energy orbital. When the electrons in these atoms absorb the energy and jump to a higher energy state it tends to lose its stability.
2. To regain stability, the electron needs to leave the excited state and fall back down from the higher excited state to the previous lower state in the form of a photon. 
3. To bring off this task, these molecules or atoms emit radiation that falls in variegated regions of the electromagnetic spectrum.
4. This spectrum of electromagnetic radiation is known as an emission spectrum.

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Absorption Spectrum

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• When a ray of white light falls on a prism it is observed to suffer refraction twice.
• The first refraction occurs when it travels from the rarer medium (air) to a denser medium (glass) and the second refraction occurs again when it travels from the denser medium (glass) to a rarer medium (air).
• Finally, we get to see a band/spectrum of colours that have resulted from a ray of white light. In this spectrum, the colour having a larger wavelength deviates the least and vice versa.
• Hence we get to see a spectrum of colours varying from violet to red and it is observed that violet suffers the most deviation while having the smaller wavelength.
• This spectrum is known to us as a continuous spectrum as violet amalgamated into blue, blue merges into green and the process continues till Red.

However, in the emission spectrum of atoms in the gas phase, the emitted light comprises a distinct wavelength with dark spaces existing between them. This spectrum is known as atomic spectra.

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Highlights on Absorption Spectrum

• A photographic negative of an emission spectrum is Known as an Absorption Spectrum.
• On a sample that absorbs radiation of certain wavelengths, electromagnetic radiations are blitzkrieg to observe the Absorption Spectrum.
• The contribution to the missing wavelength which leaves dark spaces in the bright continuous spectrum is done by the wavelength of radiation absorbed by the matter.
• The study of the absorption spectrum or emission spectrum is known as spectroscopy.

To minimize the atom's energy the electrons are arranged in a well-balanced way. The ground state of an atom is the lowest and stable energy state. The electron energy levels in atoms are quantified which means that the electron rather than travelling from one energy level to another must travel in discrete steps.

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Where the potential energy of an atom is greater than the state of the earth is called the excited state of that atom. In this state an atom is unstable. For attaining stability an atom has to go back to the ground state. In doing that it releases energy in the form of electromagnetic radiation.

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Hydrogen Absorption Series and Emission Series

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The hydrogen Spectrum is significant since the bulk portion of the Universe is made of hydrogen. The procedure of hydrogen absorption and emission give rise to series, each starting and ending with the same hydrogen atomic state. These are the series which are in a sequence of lines corresponding to atomic transitions. As an example, the Lyman Series involves transitions beginning or ending with the ground state of hydrogen, while the Balmer series includes transitions beginning (for absorption) or ending (for emission) with the first excited state of hydrogen. 

This shows levels of energy of electrons for the hydrogen atom. The arrows depict the transition of an electron from higher energy levels to lower energy levels. The absorption series can be imagined to be the opposite of the absorption spectrum.

The Lyman series is in the UV while the Balmer Series is in the visible spectrum. Transitions in the Balmer series are illustrated in the following illustration.

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The Swedish spectroscopist, Johannes Rydberg, noted that all series of lines in the hydrogen spectrum could be described by the following expression:

or

R= Rydberg constant.

n₁ = 1,2,…..

n₂= n1+1, n1+2,…….

The first five series of lines that correspond to n1 = 1, 2, 3,4, 5 are known as Lyman, Balmer, Paschen, Brackett and Pfund series, respectively.

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Things to Remember

• Emission Spectrum is defined as the spectrum of frequencies of Electromagnetic Radiation that is emitted while making a transition from a higher energy state to lower energy by an atom or molecule.
• During the interaction of electromagnetic radiation with molecules of matter or atoms, the energy helps the electron and push it to a higher energy orbital. When the electrons in these atoms absorb the energy and jump to a higher energy state it tends to lose its stability.
• To regain stability, the electron needs to leave the excited state and fall back down from the higher excited state to the previous lower state in the form of a photon.
• When a ray of white light falls on a prism it is observed to suffer refraction twice.
• The first refraction occurs when it travels from the rarer medium (air) to a denser medium (glass) and the second refraction occurs again when it travels from the denser medium (glass) to a rarer medium (air).
• The hydrogen Spectrum is significant since the bulk portion of the Universe is made of hydrogen.
• The procedure of hydrogen absorption and emission give rise to series, each starting and ending with the same hydrogen atomic state.
• These are the series which are in a sequence of lines corresponding to atomic transitions.

Read More: