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 come back to their original states. This phenomenon accounts for emission spectrum through hydrogen too, better known as hydrogen emission spectrum.
In the late 1800’s, 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 of 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 an atomic fingerprint which resulted from the internal structure of the atom.
The hydrogen spectrum is an important piece of evidence showing that the electronic structure of the atom is quantized. When an electric discharge is passed through 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 radiation of discrete frequencies. These series of radiation 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 undergoes a transition from a higher energy level to a lower one (n = 3 n = 2, for example). During this transition from a higher level to a lower level, there is the 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 emission spectrum series:
In the year 1885, on the basis of experimental observations, Balmer proposed the formula for correlating the wave number of the spectral lines emitted and the energy shells involved. This formula is given as:
This series of hydrogen emission spectrum is known as Balmer series. This is the only series of lines in the electromagnetic spectrum that lies in the visible region. The value, 109,677 cm-1, is called the Rydberg constant for hydrogen. The Balmer series is basically the part of hydrogen emission spectrum responsible for the excitation of an electron from the second shell to any other shell. Similarly, other transitions also have their own series names. Some of them are listed below,
- Transition from the first shell to any other shell – Lyman series
- Transition from the second shell to any other shell – Balmer series
- Transition from the third shell to any other shell – Paschen series
- Transition from the fourth shell to any other shell – Bracket series
- Transition from the fifth shell to any other shell – Pfund series
Johannes Rydberg, a Swedish spectroscopist, derived a general formula for the calculation of wave number of hydrogen spectral line emissions due to the transition of an electron from one orbit to another. The general formula for hydrogen emission spectrum is given by:
n1 = 1,2,3,4 …
n2 = n1 +1
ν= wave number of the electromagnetic radiation. The value 109,677 cm-1 is known as Rydberg constant for hydrogen.
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