Emission Spectrum & Absorption Spectrum
The measurement of atoms (and ions) by their interaction with electromagnetic radiation in atomic spectra is an important topic. The coloured lines of light emitted by the excited atoms form the atomic spectra. We all know about light refraction. Light either bends toward the normal or away from the normal as it passes from one medium to another. The refraction effect is primarily due to the difference in the speed of light in different mediums. The speed of light depends upon the density of the medium it travels through.
Let us understand the phenomenon of white light dispersion through a prism, and about the continuum of emissions and the spectrum of absorption.
Emission Spectrum
Electrons shift from lower energy levels to higher energy levels as energy is consumed by an atom's electrons. To return to ground states from the excited state, which is unstable, these excited electrons have to radiate energy. The spectrum of pollution is formed by the frequency of the light emitted.
The electrons in these atoms will absorb energy as electromagnetic radiation interacts with atoms and molecules of matter, and leap to a higher energy state, losing their stability.
They need to switch from the higher energy state to the preceding lower energy state in order to recover their equilibrium.
These atoms and molecules emit radiation in different regions of the electromagnetic spectrum to accomplish this task.
This radiation spectrum emitted by electrons in the excited atoms or molecules is known as the spectrum of emissions.
Absorption Spectrum
On the other hand, as energy is absorbed by electrons in the ground state to enter higher energy states, the absorption range is constituted by the frequencies of light emitted by dark bands.
We observe that it experiences refraction twice when a beam of white light falls on a prism.
Once from a rare medium (air) to a denser medium (glass) and again from a denser medium (glass) to a rare medium (glass) (air).
Finally, we see a band of colors, called the continuum, which is built out of a white light beam. The color with a smaller wavelength deviates the most as we study this spectrum more closely and vice versa.
Thus red with the longest wavelength suffers the least variance, a continuum of colors ranging from red to violet is observed.
As violet merges into blue, blue into green and so on, this sort of spectrum is called a continuous spectrum.
However, in the gas phase, the emission spectrum of atoms does not display a constant spread of wavelengths from one color to another. Instead, the light emitted consists of a single wavelength with dark spaces occurring between them. These kinds of spectra are referred to as spectra of atoms or line spectra.
Emission Spectrum & Absorption Spectrum
In an atom, the electrons appear to be arranged in such a way that the atom's energy is as minimal as possible. The lowest energy state of the atom is the ground state of an atom. The electrons consume the energy and transfer to a higher energy level when those atoms are given energy. These electron energy levels in atoms are quantified, meaning again that the electron must travel in discrete steps rather than constantly from one energy level to another.
An atom's excited state is a state where its potential energy is greater than the state of the earth. An atom is not stable in a state of excitation. As it goes back to the ground state, in the form of electromagnetic radiation, it releases the energy it had previously accumulated.
An absorption spectrum is like a negative photograph of a spectrum of pollution.
Electromagnetic radiation is bombarded with a sample that absorbs radiation from certain wavelengths to observe the absorption spectrum.
Hydrogen Emission and Absorption Series
In astronomy, the spectrum of hydrogen is especially significant since much of the Universe is made of hydrogen. The processes of hydrogen emission or absorption give rise to series, which are sequences of lines corresponding to atomic transitions, each ending or starting with the same hydrogen atomic state. The Balmer Series, for example, includes transitions beginning (for absorption) or ending (for emission) with the first excited state of hydrogen, while the Lyman Series involves transitions beginning or ending with hydrogen's ground state; the adjacent picture shows the atomic transitions that create emissions from these two series.
The Balmer Series is in the visible spectrum and the Lyman Series is in the UV, owing to the specifics of hydrogen's atomic structure. Some of the transitions in the Balmer series are illustrated in the following illustration.
Molecular Spectra
Molecules can interact with electromagnetic radiation and give rise to characteristic spectra, in addition to spectra associated with atoms and ions. Because of the fundamental atomic and molecular structure, infrared wavelengths usually provide the spectra associated with molecules. Moreover, since molecules are typically fragile, molecular spectra are mainly essential in relatively cold objects, such as planetary atmospheres, very cool star surfaces, and different interstellar regions.
FAQs on Emission Spectrum and Atomic Spectra
1. What is an atomic spectrum?
An atomic spectrum is the unique pattern of electromagnetic radiation that is absorbed or emitted by an atom. When an element is excited by energy (like heat or electricity), its electrons jump to higher energy levels and then fall back, releasing light at specific wavelengths. This light, when passed through a prism, does not form a continuous rainbow but rather a series of distinct, bright lines against a dark background. This unique set of lines is the element's atomic emission spectrum.
2. What is the fundamental difference between an emission spectrum and an absorption spectrum?
The fundamental difference lies in how they are produced and what they look like.
- An emission spectrum is created when an atom releases energy. It appears as a series of bright, coloured lines on a dark background. Each line corresponds to a specific frequency of light emitted by an electron transitioning to a lower energy level.
- An absorption spectrum is created when an atom absorbs energy from a continuous light source. It appears as a continuous spectrum (like a rainbow) with dark lines or gaps. These dark lines represent the specific frequencies of light that were absorbed by the atom's electrons to jump to higher energy levels.
3. How is a line spectrum different from a continuous spectrum?
A continuous spectrum, like the one from a rainbow or a light bulb, contains all wavelengths of light within a given range without any gaps. The colours blend smoothly into one another. In contrast, a line spectrum (or atomic spectrum) consists of only a few distinct, sharp lines of specific wavelengths. This occurs because electrons in atoms can only exist in discrete, quantised energy levels and can only emit or absorb photons of specific energies corresponding to the differences between these levels.
4. Why is the atomic spectrum of an element often called its 'fingerprint'?
The atomic spectrum of an element is called its 'fingerprint' because it is completely unique to that element. Every element has a distinct number of protons and a unique arrangement of electrons in its energy levels. Due to this unique electronic structure, the wavelengths of light it can emit or absorb are also unique. Just as no two people have the same fingerprint, no two elements have the same atomic spectrum. This property is fundamental to the field of spectroscopy.
5. How does the study of atomic spectra help in identifying unknown elements, for example, in distant stars?
Astronomers use atomic spectra to determine the chemical composition of distant celestial objects like stars. Light from a star travels through its outer atmosphere, where cooler gases absorb specific wavelengths of light. When this starlight is analysed on Earth, it shows an absorption spectrum. By matching the pattern of dark lines in this spectrum to the known absorption spectra of elements measured in a lab, scientists can precisely identify which elements are present in the star's atmosphere. This technique, called astronomical spectroscopy, is how helium was first discovered on the Sun before it was found on Earth.
6. What are the main spectral series observed in the hydrogen atom's spectrum?
The emission spectrum of hydrogen is categorised into several series, each named after its discoverer. Each series corresponds to electrons transitioning to a specific final energy level (n). The most prominent series are:
- Lyman Series: Electrons transition to the ground state (n=1). These lines are in the ultraviolet region.
- Balmer Series: Electrons transition to the first excited state (n=2). These lines are primarily in the visible light region.
- Paschen Series: Electrons transition to the second excited state (n=3). These lines are in the infrared region.
- Brackett Series: Electrons transition to the third excited state (n=4).
- Pfund Series: Electrons transition to the fourth excited state (n=5).
7. How did Bohr's atomic model explain the formation of discrete lines in the hydrogen spectrum?
Bohr's model was revolutionary because it successfully explained the line spectrum of hydrogen. It proposed that electrons orbit the nucleus in fixed, quantised energy shells or orbits, not just anywhere. An electron can only move between these specific orbits by absorbing or emitting a precise amount of energy (a photon). The energy of this photon is exactly equal to the energy difference between the two orbits. Since there are only specific, allowed energy differences, only photons of specific frequencies are emitted, resulting in the sharp, discrete lines observed in the hydrogen spectrum rather than a continuous blur.
8. What is the key difference between atomic spectra and molecular spectra?
The key difference is complexity. Atomic spectra arise from electronic transitions within individual atoms and produce simple line spectra. Molecular spectra are much more complex because, in addition to electronic transitions, molecules can also absorb energy to undergo vibrational and rotational changes. These additional energy levels are very close together, causing the sharp atomic lines to broaden into bands. Therefore, molecular spectra appear as bands of closely spaced lines, especially in the infrared region, rather than the distinct, separate lines of atomic spectra.
