Hydrogen spectrum-Hydrogen emission spectrum-chemistry learners
Hydrogen spectrum
Hydrogen spectrum introduction
You are a student preparing for your term-end exams or a job seeker preparing for competitive exams with chemistry as one subject. You will see the atomic structure concept in your exam syllabus for sure. And in it, hydrogen spectrum is the most common word you come across in the first line. Then this blog will help you explore the insights of the hydrogen spectrum. In this article, we will cover concepts like what is the hydrogen spectrum? Types of hydrogen spectrum? What is the hydrogen emission spectrum? What is the hydrogen absorption spectrum? So, without further delay, let us discuss those things quickly.
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| A diagram representing various elements of the hydrogen spectrum |
What is meant by hydrogen spectrum?
The hydrogen spectrum substantiates the existence of the quantized electronic structure of an atom. In ordinary conditions, the hydrogen atoms combine to form hydrogen molecules. The electric current that passes dissociates the molecules to form hydrogen atoms.
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| The dissociation of the hydrogen molecule in the presence of an electric discharge |
The transition of electrons between the two stationary energy levels with absorption immediately followed by the emission of integrated bundles of quantum results in the appearance of spectral lines in the hydrogen spectrum. Each spectral line corresponds to the emitted photons of discrete frequencies. And the sequence of radiations released gives the series of hydrogen spectrum named after the scientists discovered them.
History of the hydrogen spectrum
Johann Jakob Balmer discovered an empirical formula in 1885 to analyze the spectral lines of the hydrogen atom in the visible region. In 1888, Rydberg generalized this Balmer formula to calculate the wavelength of spectral lines corresponding to all the electron transitions of the hydrogen atom. In 1913, Neil Bohr's atomic model elucidated the spectral lines in the hydrogen spectrum and determined the hydrogen spectrum formula.
Each spectral line corresponds to the wavelength of an electromagnetic radiation emitted or absorbed by the hydrogen atom. And the Rydberg formula helps to determine the wavelengths of these spectral lines in the line spectrum of hydrogen. Additionally, the hydrogen spectrum has a prominent place in astronomy due to hydrogen abundance in the universe.
Overview of the spectrum and its types
Spectrum is an arrangement of radiations of various wavelengths obtained by passing polychromatic radiation through the prism.
Types of spectra
There are two types of hydrogen spectra- Emission spectrum
- Absorption spectrum
Emission spectrum
It is a specimen of bright lines or bands obtained by passing emitted polychromatic light rays through the prism.
Absorption spectrum
It is a pattern of dark lines or bands attained by transmitting an absorbed light beam into the spectroscope.
Not to mention the spectrum of hydrogen is also of two types;
- Hydrogen emission spectrum
- Hydrogen absorption spectrum
Bohr explanation of hydrogen spectrum
According to Neil Bohr, each atom consists of a central nucleus with revolving electrons in stationary states. And these stationary states associate with a definite amount of energy and are hence termed as energy shells.
He emphasized that the electron neither absorbs energy nor emits it as long as it revolves in a particular stationary state.
When the electron intakes either thermal energy or electric energy from an external source, it excites from the ground state to a higher energy state of the atom. Since being unstable, the hydrogen electron returns to its initial lower energy state with the emission of photons of suitable wavelength. It results in the emergence of spectral lines in the atomic spectrum of hydrogen. And accordingly, these spectral lines are known as hydrogen spectrum lines.
Moreover, these individually distinct spectral lines relate to the sporadic photon emissions by the energy quantization principle mentioned by Bohr. And hence, the spectrum of hydrogen is also known as the line spectrum.
Additional reference:
The hydrogen emission spectrum
Experimental setup for the emission spectrum of hydrogen
Consider a sample of hydrogen gas in the glass discharge tube. The electric current is passed through the hydrogen gas present in the discharge tube under low pressure. When the hydrogen atoms absorb energy from the electric discharge, they get excited to higher energy states. And the unsettled electron in the excited state then returns to its initial position with the emission of photons of suitable wavelengths.
Now, the hydrogen gas in the discharge tube glows red indicating the electron transition between the two different energy levels. And the emitted light radiations is passed through the slit and made to fall on the glass prism that separates the light radiation into constituent wavelengths. Finally, the photographic plate placed over there records the line emission spectrum of hydrogen.
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| The hydrogen emission spectrum diagram |
Additional reference:
Observations in the emission spectrum of hydrogen
The hydrogen spectrum contains a set of lines in the ultraviolet, visible and infrared regions. And the wavelengths of lines obtained below 400nm falls in the ultraviolet part of the electromagnetic spectrum. Similarly, wavelengths of lines obtained above 700nm are in the infrared zone. The spectral lines in the visible region have wavelengths between 400-700nm. The different wavelengths of light energy produced by hydrogen atoms are also known as the hydrogen light spectrum. |
| The electromagnetic spectrum |
The colored spectral emissions in the visible region on the photographic plate are noticeable with the human eye. Besides, special spectrographic techniques are essential to observe the spectral lines in the UV and IR regions.
The objects, when heated sufficiently, emit photons of suitable wavelengths in the electromagnetic spectrum. At low temperature, the body radiates red light that falls in the low frequency region of the visible spectrum. At high temperatures, it emits violet-colored light radiations at high frequencies. The bodies emit radiations continuously at extreme temperatures by classical quantum mechanics.
But such continuous spectral emissions like solar spectrum are not observed for hydrogen atoms even at high temperatures. It is, however, clarified by the scientist Max Planck in his quantum theory of radiations.
According to him, the harmonic oscillator emits an integral bundles of energy known as quantum rather than emitting erratic energies.
In other words, the body can transfer integral bundles of energy. And it varies directly with the frequency of radiation in the electromagnetic spectrum.
So the emission of light radiations at specific frequencies indicates only fixed values of energy are absorbed by the body discontinuously. It accounts for the appearance of a discontinuous spectrum for hydrogen.
Have a look at ππ
An interesting image on hydrogen color spectrum
@chemistrylearners, on Feb 15
In the visible zone, four different spectral lines observed at wavelengths 656nm, 486nm, 434nm and 410nm give characteristic red, aqua, blue and violet-colored lines in the visible emission spectrum of hydrogen. Thus, the hydrogen color spectrum is the range of different wavelengths in the visible region of the hydrogen emission spectrum.
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| The hydrogen color spectrum |
But on passage of electric current through hydrogen gas in the vacuum tube, 12 spectral lines are observed in the Balmer series. The occurrence of more spectral lines justifies the assumption of electron transitions from the ground state to an infinite number of excited states.
Moreover, the spectrum of celestial bodies gave 33 spectral lines in the Balmer series. The fundamental assumption for more spectral lines was the low density of hydrogen gas in astronomical bodies. But it was discarded by vacuum tube experiment on hydrogen for spectral lines corresponding to higher energy states in the Balmer series. However, the absorption spectrum of hydrogen atoms revealed this uncertainty.
Additional reference:
Fowler experiment overview
Fowler succeeded in obtaining the spectrum of cosmic hydrogen by experiments with vacuum tubes containing a mixture of hydrogen and helium.
A neutral helium atom consists of a nucleus with two units of positive charge surrounded by two revolving electrons. And during the formation of the system, the binding energy of a single electron by the helium nucleus can be E=2e. So the amount of energy emitted is equal to;
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| The Fowler formula expresses the frequency of the helium nucleus. |
Observations
He observed a series of lines in the extreme ultraviolet region during electron transitions from the second orbit to the ground state. And two spectral sequences named the first and second principal series observed during the electron movement from the third orbit to its initial lower energy position. This spectral series is similar to Pickering in the spectrum of ΞΆ puppies found by the electron transition from higher energy state n=4. Fowler called these high intensity spectral sequences as sharp series of the hydrogen spectrum. And his experiments gave evidence for the presence of hydrogen in the stars.
The study of spectral lines is very complicated for the atom containing more than one electron. Consequently, the difference in the spatial arrangement of electrons from various stationary orbits gives different sets of spectral lines in the emission spectrum of the given substance. And it did not take inter-electronic interactions into account. Therefore, Bohr atomic model could not explain the emission spectra of multi-electron species.
Rydberg studied the frequency of spectral lines emitted from hydrogen atom during electron transition, And according to him,
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| The Rydberg formula for the hydrogen emission spectrum |
"The sum or difference of frequencies of existing spectral lines determines the frequency of unknown spectral lines of that element."
Based on this principle, Ritz found the wavenumber of spectral lines with the following expression.
And it closely agrees with those calculated Rydberg and Ritz. It proves the reliability of the Bohr's atomic model for the hydrogen spectrum.
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| The Ritz formula for the hydrogen emission spectrum |
Here either n1 or n2 is a fixed number and, the other fluctuates. It gives a sequence of transition lines for the atoms similar to hydrogen. Ritz successfully identified those atomic spectral lines with his combination principle.
Meanwhile, Bohr experimentally calculated the value of the factor outside the bracket.
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| Bohr calculation |
The hydrogen absorption spectrum
Experimental setup for the absorption spectrum of hydrogen
Consider a sample of hydrogen gas in the glass discharge tube. A white light passes through the vapors or solution of the hydrogen gas in the discharge tube. And the transmitted light then passes through the spectroscope gives a spectrum of dark lines at definite wavelengths. These dark lines correspond to the wavelengths of light radiations absorbed by the hydrogen atoms. Hence, we observe dark lines in the absorption spectrum of hydrogen on continuous white background.
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| The diagram of the hydrogen absorption spectrum |
As everyone knows, the hydrogen atom consists of a nucleus at its center with a single electron revolving around it. Let n1 and n2 are the two stationary states where n2>n1. The electron must present at n1 level to absorb energy from light radiation. And when the transmitted light is allowed to pass through the prism gives the dark line absorption spectrum of the hydrogen atom.
The line absorption spectrum of atomic hydrogen involves two kinds of electron transitions. They are;
- Electron transitions from the ground level to a higher excited state
- Electron transitions from the ground state to a state of an atom in which the electron is free
Case-1: Electron transitions from the ground state to a higher excited state
When the electron passes between the two successive states of the hydrogen atom gives spectral emissions. The wavelength of spectral lines obtained in the spectrum depends upon the energy of the light radiation absorbed by the substance.
By the quantum theory of radiation, each quantum possesses a definite amount of energy depending upon the frequency of radiation. Hence, the quantum energy varies directly with the frequency. The following mathematical equation expresses it.
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| Energy-frequency relationship by the quantum theory |
Again, the frequency of light is inversely proportional to its wavelength.
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| Frequency-wavelength relationship |
Where, c is the velocity of the light radiation in vacuum
The above energy equation can be rewritten as;
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| Calculation of energy expressed in terms of wavenumber |
Case-2: Electron transitions from the ground state to a state of an atom in which the electron is free
In the free state, the electron is at an infinite distance from the nucleus by using its kinetic energy. The difference between the linear dimensions of orbits and electron frequencies diminishes in stationary states with a higher n value. Moreover, the energy emitted by the electron in the free state is not significant. Hence, the total energy of the system is assumed to be constant. In this state, the absorption of light energy causes the ejection of an electron from the atom. This phenomenon is observed in experiments on ionization by ultraviolet light and Rontgen rays. And the kinetic energy of an ejected electron can be calculated by;
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| The equation shows the kinetic energy of the ejected electron. |
T= Kinetic energy of the ejected electron
W= The amount of energy emitted during the formation of the system
Justification for unknown spectral lines in the Balmer series of the hydrogen spectrum
The emission spectrum of hydrogen gas in the vacuum tube gives 12 lines in the Balmer series. But, the celestial bodies show 33 lines in the Balmer series. The absorption spectrum of hydrogen reveals the difference in their spectral emissions.
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| Difference between the spectral emissions of the hydrogen in the vacuum tube and the astronomical bodies. |
The R.W.Wood experiments on sodium vapor absorption spectrum explain the reason behind the spectral emissions of astronomical bodies.
According to him,
"The orbital radius is large at higher n values than those close to the nucleus. So the electron transitions at infinite stationary levels give more spectral lines. This kind of electron transition does not occur in vacuum tubes."
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| Differences in spectral emissions of the hydrogen atom |
Consequently, it serves as a reason for the nonappearance of spectral lines corresponding to higher n values in the Balmer series of hydrogen spectrum in vacuum tubes. Hence, the hydrogen atoms in the vacuum tube show only 12 spectral lines in the Balmer series.
Additional reference:
Different scenarios for absorption and scattering of light
In case of no collision between the electrons of stationary systems scatters the light radiation absorbed by it formerly. Here, the frequency of the absorbed and emitted photon is the same by Kirchhoff's law.
The bound electron scatters absorbed homogeneous radiation when the frequency of light is greater than the factor W/h. For free electron the scattering of absorbed light radiation is solely dependent on the frequency of emitted photon.
Absorption involves the collision between the particles of the system. And a part of the absorbed radiant energy is transformed into the kinetic energy of colliding electrons. During the collision, the bound electron acquires energy equal to the energy difference between the two successive stationary states of the system.
When cathode rays pass through the atom, one or more bound electrons detaches from it. The fast moving electron rays collide with innermost electrons by transferring a finite quantum of energy. And it is equal to the energy difference between the two successive stationary states of the system. A core hole is observed in the shell where the electron is left. And another electron from the outermost shell then falls into the core hole by losing energy with the emission of characteristic X-rays.
Applications of the hydrogen spectrum
Many street lights use bulbs that contain sodium or mercury vapor. On absorbing energy, the electrons get excited to a higher energy state and get back to their original level by emitting energy. In the case of sodium, most emission lines are at 450nm giving a characteristic yellow light. And for mercury, the intense emissions are at 589nm gives a blue-colored light.
Each atom has its characteristic emission and absorption spectra. It serves as a fingerprint to characterize them. The spectroscopic data of known substances serves as a reference to reveal spectroscopic data of unknown bodies. Hence it helps in the identification of new substances in the universe.
Questions and answers on the hydrogen spectrum concept
1. Why is the hydrogen spectrum not a continuous spectrum?
The emission spectrum of hydrogen is discontinuous. Since the hydrogen emission spectrum consists of a series of sharp lines separated by dark bands. These individually distinct spectral lines relate to the sporadic photon emissions by the quantization of energy mentioned by Bohr.
2. Why is the hydrogen spectrum called the line spectrum?
The absorption of electric energy excites the hydrogen electron from the ground state to one of the higher energy orbits. After being unstable, the electron bounces back to its initial position abruptly with the emission of photons of discrete frequencies. It gives a spectral line in the hydrogen spectrum. These individually distinct spectral lines relate to the sporadic photon emissions by the energy quantization principle mentioned by Bohr. Hence, the hydrogen spectrum is discontinuous with a series of sharp lines separated by dark bands is also known as a line spectrum.
3. Why are the low pressure and high voltage conditions maintained in the discharge tube for the hydrogen emission spectrum?
Hydrogen is the lightest element in the periodic table with a high charge to mass ratio upon ionization. And we all know gases are poor conductors of electricity. The gases can conduct electricity only under two circumstances;
- By applying high voltage condition
- By maintaining low pressure condition
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| The image shows the laboratory setup for the hydrogen emission spectrum. |
In ordinary conditions, the hydrogen atoms combine to form hydrogen molecules. The electric current that passes dissociates the molecules to form hydrogen atoms. The voltage is essential for the flow of electric current that causes the ionization of gas molecules.
The minimum amount of energy required to move the electron farther away from the nucleus is ionization energy. And the process is known as ionization.
The kinetic energy of electrons that flows from the cathode increases at high voltage. Hence, the energetic electrons collide with gaseous atoms to cause excitation.
After being unstable, the excited atoms return to the ground state with the emission of photons of suitable frequencies. It gives the hydrogen emission spectrum with a series of spectral lines based on their wavelengths.
4. What is a discharge tube? Why is it called a discharge tube?
A discharge tube is an electrochemical cell that releases and accepts electrons at its electrodes. It is an evacuated glass tube that has metal electrodes at its ends. These electrodes act as cathode and anode. And they are connected to the terminates of the induction coil. A vacuum pump is connected to the discharge tube to maintain low pressure conditions in the discharge tube.
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| The image is the diagrammatic representation of the discharge tube. |
The gas contained in the discharge tube undergoes ionization by the induced electric field. It causes the excitation of gas atoms which then return to their original position with the emission of light energy.
Last but not the least, it is a device used to study the conduction of electric current through the gases under low pressure conditions with the emission of light energy. And hence it is called a discharge tube.
What additional aspects do you know about the hydrogen spectrum?
We have discussed the hydrogen spectrum definition, hydrogen emission and absorption spectrums in detail. We all know the hydrogen atom has a single electron in the n=1 state. It should only give Lyman series. But how the other series of hydrogen spectrum is possible. And how a single electron manages to show an infinite number of spectral lines in the hydrogen spectrum.
No worries, I am discussing it now. The hydrogen atom has a single electron that transits between the infinite numbers of stationary orbits of the hydrogen atom. The electron is not stable in the n=1 state. On absorbing energy, the electron moves to one of the higher energy orbits and returns to its initial state abruptly with the emission of photons.
The quantum emission moves the electron to its destination position either directly in a single step or through several intermediate steps. It depends upon the amount of energy involved in the transition. It is the sole reason for the appearance of different transition series in both the hydrogen emission and absorption spectrums. And also for the occurrence of the infinite number of spectral lines in the hydrogen spectrum. Moreover, the position of emission lines is the same as that of absorption lines. Hence, we get identical spectral line data in both emission and absorption spectrums. All the chemical elements give their emission and absorption spectrums along with the hydrogen atom.
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