A spectrum is a band of different colors produced by radiation or of light (energy). It is either a continuous or a separated sequence of a wide range of colors produced by light when it passes from one medium to another. The most common example of such a spectrum is a rainbow.
In chemistry, however, a spectrum is a range of characteristic wavelengths of electromagnetic radiation emitted or absorbed by an atom or a molecule. Such spectrums are studied in atomic emission spectroscopy and atomic absorption spectroscopy respectively.
The continuous spectrum has no gaps, breaks, or sharp lines in a given range of frequencies or wavelengths while the line spectrum contains clear gaps or lines between colored regions. A continuous spectrum is a combination of the absorption spectrum and emission spectrum, on the other hand, a line spectrum has either emission or an absorption-based spectrum.
Continuous vs Line Spectrum
| Continuous spectrum | Line spectrum |
| Continuous spectrum has all wavelengths within a certain range | Line spectrum contains only a few wavelengths within a certain range |
| It contains non-observable gaps | It contains huge gaps between lines or bands |
| It is the combination of the absorption spectrum and emission spectrum | Line spectrum is either absorption or emission spectrum |
| Example of this type of spectrum is a rainbow | Emission and absorption spectrum of a hydrogen atom is an example of line spectrum |
The Electro-Magnetic (EMR) Spectrum
The electromagnetic spectrum consists of an enormous range of frequencies and wavelengths. In this range of electromagnetic radiations (EMR), the radiations of different energies interact with chemical species to bring a variety of changes in them.
The record of the intensity of light (radiations) transmitted or scattered by an atom or a molecule as a fraction of its frequency, wavelength, or wavenumber is called its spectrum (From Latin words ‘spectre‘ and ‘apparition‘ meaning ‘appearance’).
The interaction between electromagnetic energy and matter is studied in the field of spectroscopy. When light of a certain wavelength passes through an object, some of it can be absorbed and the rest can be emitted at a different wavelength from its original one. Optical instruments called spectrometers reveal a series of specific wavelengths or frequencies in which the light energy is absorbed and emitted. These records are generally referred to as spectra.
Different types of spectrometry and spectroscopy techniques are listed as:
- Nuclear Magnetic Resonance (NMR)
- Electron Spin Resonance (ESR)
- Microwave spectroscopy
- Infrared spectroscopy
- UV-Vis Spectroscopy
- X-ray spectroscopy
- Gamma spectroscopy
- Mass Spectrometry (MS)
Before the interaction of electromagnetic radiations, the analyte is in the ground state. When EMR of different energies interact with that analyte, some of the energy is absorbed and the chemical species undergo a transition to a higher (excited) state.
The information about the analyte is obtained from the amount of EMR absorbed as a result of excitation or measuring the electromagnetic radiation emitted when the analyte turns back to the ground state from the excited state.
Absorption spectrum
In the absorption spectrum, the electromagnetic radiation of certain frequencies is transmitted through an analyte with dark fringes or bands when the ground-state electrons absorb energy to get to higher levels or excited.
When electromagnetic radiation is passed through chemical species, certain wavelengths are absorbed. The absorbed wavelengths leave dark spaces in the continuous spectrum. Every atom has its unique absorption and emission spectrum range because the energy difference is discrete and constant. It means that all atoms always absorb the same characteristic wavelength from given electromagnetic radiation.
The absorption spectrum is plotted with absorbance against wavelength or frequency. The analysis of that spectra gives information about the identity and concentration of a specific substance.
Emission spectrum
The emission or bright-line spectrum of chemical species comprises a range of frequencies of electromagnetic radiation emitted due to the transition of an electron from its excited (high-energy)state to the ground (low-energy) state.
The emission spectrum is opposite to the absorption spectrum. For a given atom, the absorbed lines in absorption spectra and emitted lines in emission spectra are in the same place. This is because the absorbed energy is the same as radiant energy.
Continuous spectrum
In 1672, Isaac Newton was the first to discover that sunlight is made up of multiple frequencies or wavelengths. He separated sunlight into its component colors by using prisms and showed that sunlight consists of a continuous array of colors. Through this, the VIBGYOR (violet, indigo, blue, green, yellow, orange, red) diversity sequence came to be known.
A continuous spectrum has no line separation between two different colors yet they form a gradient. It means that this spectrum has no visible boundaries or gaps between a range of frequencies or wavelengths.
Line spectrum
Line spectrum occurs when wavelengths of radiation emitted or absorbed by a substance are well separated. The individual particles in the gas medium behave independently of one another. Thus the spectrum in most media consists of a series of sharp lines with widths of 10-1 – 10-2 Å (10-2 – 10-3 nm).
The line spectrum shows only a certain wavelength within a given range. These wavelengths are very sharp lines with well-defined boundaries. This type of spectra would be either emission or absorption spectrum.
Spectra of a Hydrogen Atom
In 1890, Swedish spectroscopist Johannes Rydberg described the spectra of hydrogen atoms. It was all a follow-up of Bohr’s atomic model and through his explanation, scientists were able to develop an atomic theory with the least limitations.
When an electric discharge is passed through hydrogen gas (H2), the molecules excite to form hydrogen atoms and emit light (radiation) of the discrete frequencies. These frequencies produce a spectrum of a series of lines. The spectrum of a hydrogen atom is important to understand that energy levels of electrons are quantized.
Rydberg gave an expression to describe the wave number of emitted spectra;
where;
- ν = wave number of the emitted radiation
- RH = Rydberg’s constant (1.097 x 107 m-1)
- n1 = Shell where electron arrives
- n2 = Shell from where electron leaves
When a photon is absorbed by a hydrogen atom, the absorbed energy cause excitation of an electron from a low energy level to a high energy level. On the other hand, when an electron jumps from a higher energy level to a lower energy level, the hydrogen atom emits a photon. Since energy levels are quantized, so certain brands or lines appear on the spectrum.
When an electron jumps from any higher orbit to the first orbit, it is termed the Lyman series. Similarly, Balmer, Paschen, Brackett, and Pfund series are for electrons that come from higher orbits to second, third, fourth, and fifth orbits respectively.
Key Takeaway(s)
Concepts Berg
What is a continuous spectrum?
A continuous spectrum is an emission spectrum that consists of all the wavelengths of a given range. It has no line character, so it does not comprise any type of gap.
What is an absorption spectrum?
In absorption spectrum, the electromagnetic radiation of certain frequencies is transmitted through an analyte with dark fringes or bands when the ground-state electron absorbs energy to get excited.
What is an emission spectrum?
The emission or bright-line spectrum of a chemical species comprises a range of frequencies of electromagnetic radiation emitted due to the transition of electrons from an excited state to the ground state.
What is a line spectrum?
Line spectrum occurs when wavelengths of radiation emitted or absorbed by a substance are well separated.
Why is the line spectrum not continuous?
Line spectrum does not contain all wavelengths of a given range as in continuous spectra. This is because every individual particle behaves independently throughout the medium and the energy level of electrons is quantized.
Examples of line spectrum and continuous spectrum:
Example of line spectrum:
- Emission spectrum and absorption spectrum of hydrogen gas.
Example of the continuous spectrum:
- Rainbow and blackbody radiations.
What are the types of line spectrum?
There are two types of line spectrum;
- Emission spectrum
- Absorption spectrum
What is a discontinuous spectrum?
A discontinuous spectrum contains gaps, breaks, or well-defined boundaries corresponding to the wavelengths. Line spectra is an example of discontinuous spectra.
What do the dark lines in an absorption spectrum indicate?
In an absorption spectrum, the dark lines indicate the absorbed wavelengths which are characteristics of an individual particle. All other wavelengths are emitted and show bright bands.
What is the origin of the atomic emission spectrum of an element?
The atomic emission spectrum is produced when an excited electron gets back to its ground state and gives out radiant energy which shows on the spectrum.
Where did Bohr’s model of the atom fail?
Bohr’s atomic model failed to explain the phenomena of the Zeeman and Stark effect. The Zeeman and Stark effects are splitting of spectral lines in presence of magnetic field and electric field respectively.
References
- Fundamentals of Analytical Chemistry| (Second Edition) by Douglas A. Skoog (Stanford University) and Donald M. West (San Jose State University)
