What is Emission Spectrum?
Whenever electromagnetic radiations interact with atoms and molecules of matter, the electrons in these atoms may absorb energy and jump to a higher energy state, losing their stability. To regain their stability, they need to move from the higher energy state to the previous lower energy state. To accomplish this job, these atoms and molecules emit radiations in various regions of the electromagnetic spectrum. This spectrum of radiation emitted by electrons in the excited atoms or molecules is known as the emission spectrum. It can be defined as:
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an atom or molecule transitioning from a high energy state to a lower energy state.
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Atomic Spectra
- We observe that when a ray of white light falls on a prism, it experiences refraction twice. Once when it travels from the rarer medium (air) to a denser medium (glass) and again from the denser medium (glass) to a rarer medium (air).
- Finally, we observe a band of colours, called a spectrum, formed out of a ray of white light. If we observe this spectrum more closely, the colour’s smaller wavelength deviates the most and vice versa.
- Thus, a spectrum of colours ranging from red to violet is observed where red having the longest wavelength, suffers the least deviation. This kind of spectrum is called continuous spectrum as violet merges into blue, blue into green and so on.
- However, the emission spectrum of atoms in the gas phase does not exhibit a continuous spread of wavelength from one colour to another. Rather, the emitted light consists of a specific wavelength having dark spaces existing between them. Such kind of spectra is known as atomic spectra or line spectra.
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Absorption Spectrum
Unlike the emission spectrum, an absorption spectrum is like a photographic negative of the emission spectrum. For observing the absorption spectrum, electromagnetic radiations are bombarded on a sample which absorbs radiation of certain wavelengths. The wavelength of radiations absorbed by the matter contributes to the missing wavelength, which leaves dark spaces in the bright continuous spectrum. Each element has its unique line emission spectrum. The study of the emission spectrum or absorption spectrum is better known as spectroscopy.
Hydrogen Emission Spectrum
We all know that electrons in an atom or a molecule absorb energy and get excited, they jump from a lower energy level to a higher energy level, and they emit radiation when they return to their original states. This phenomenon accounts for the emission spectrum through hydrogen, too, better known as the hydrogen emission spectrum.
- In the late 1800s, it was known that when a gas is excited using an electric discharge and the light emitted is viewed through a diffraction grating, the spectrum observed consists not of a continuous band of light but individual lines with well-defined wavelengths. Experiments have shown that the wavelengths of the lines were characteristic of the chemical element emitting the light. They were atomic fingerprints resulting from the atom’s internal structure.
- The hydrogen spectrum is important evidence showing that the atom’s electronic structure is quantized. When an electric discharge is passed through a gaseous hydrogen molecule, the hydrogen atoms in the molecule dissociate. This leads to the emission of electromagnetic radiation by the energetically excited hydrogen atoms. The hydrogen emission spectrum consists of radiations of discrete frequencies. These series of radiations are named after the scientists who discovered them.
When a photon is absorbed by a hydrogen atom, the energy of the photon causes the electron to undergo a transition to a higher energy level (n = 1 n = 2, for example). When a hydrogen atom emits a photon, the electron transitions from a higher energy level to a lower one (n = 3 n = 2, for example). During this transition from a higher to a lower level, a transmission of light occurs. Since the energy levels of the atom are quantized, the spectrum will consist of wavelengths that reflect the differences in these energy levels. For example, the line at 656 nm corresponds to the transition n = 3 n = 2.
Hydrogen Transitions
Hydrogen Emission Spectrum Series:
In the year 1885, based on experimental observations, Balmer proposed the formula for correlating the wavenumber of the spectral lines emitted and the energy shells involved.
This formula is given as:
\(\begin{array}{l}\bar{\nu }=109677(\frac{1}{2^{2}}-\frac{1}{n^{2}})\end{array} \) |
This series of the hydrogen emission spectrum is known as the Balmer series. This is the only series of the line in the electromagnetic spectrum that lies in the visible region. The term RH is called the Rydberg constant for hydrogen.
Rydberg constant is given by:
\(\begin{array}{l}R_{H}= \frac{me^{4}}{8\epsilon _{0}h^{3}c}\end{array} \) |
Where,
- h is the Planck’s constant
- c is the speed of light
Using the known value of these constants we can compute the value of Rydberg constant, RH = 109677.
The Balmer series is basically the part of the hydrogen emission spectrum which is responsible for the excitation of an electron from the second shell to any other shell. Similarly, other transitions have their own series names. Some of them are listed below,
- The transition from the first shell to any other shell – Lyman series
- The transition from the second shell to any other shell – Balmer series
- The transition from the third shell to any other shell – Paschen series
- The transition from the fourth shell to any other shell – Bracket series
- The transition from the fifth shell to any other shell – Pfund series
Johannes Rydberg, a Swedish spectroscopist, derived a general formula for calculating the wavenumber of hydrogen spectral line emissions due to the transition of the electron from one orbit to another.
The general formula for the hydrogen emission spectrum is given by:
\(\begin{array}{l}\bar{\nu }=109677(\frac{1}{n_{1}^{2}}-\frac{1}{n_{2}^{2}})\end{array} \) |
Where,
- n1 = 1,2,3,4 …
- n2 = n1 +1
- \(\begin{array}{l}\bar{\nu }\,\, \textup{= wavenumber of the electromagnetic radiation. The value 109,677}\,\, cm^{-1}\,\, \textup{is known as Rydberg constant for hydrogen.}\end{array} \)
Frequently Asked Questions – FAQs
What are the waves covered in electromagnetic spectrum?
What is the main source of ultraviolet radiation?
The sun is the primary source of ultraviolet radiation.
What is the formula to calculate the hydrogen emission spectrum?
Define Rydberg constant.
What is meant by spectroscopy?
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the above given notes are very useful thank you very much .it would be better if you use some images with the explanations