Theory of Molecular Spectra

What Do You Mean By Theory of Molecular Spectra?

The essential resource that is available for investigating the molecular structure is the theory of molecular spectra. The discrete energy states of a certain molecule can be derived directly from the theory. Molecular energy levels and molecules in an excited state might seem more complicated to understand than atomic energy levels. This is because of the reason that molecules have the ability to rotate and vibrate. The energy which is associated with such motions of rotation and vibration lie in different ranges. You can study these different ranges separately to avoid complications and understand the subject matter easily.

Understanding Spectroscopy

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Spectroscopy or molecular spectroscopy is basically the area of science that deals with the emission, absorption, and scattering of electromagnetic radiation by molecules and atoms in phases such as solid, liquid, and gas. Spectroscopy also plays a crucial role in understanding the results of spectroscopic experiments and the development of the laws of quantum mechanics. The aim of spectroscopy is to make the discussion more concrete about the quantum mechanical description of the molecular properties and the laws of quantum mechanics. 


Electromagnetic Spectrum

Before you delve into understanding molecular spectroscopy in detail, first, you will need to get a solid understanding of the subject of the electromagnetic spectrum. It is because spectroscopy is defined as the scientific area that deals with the emission, absorption, and scattering of electromagnetic radiation by molecules and atoms. Visible electromagnetic radiation is termed light, although here, you can use the terms electromagnetic radiation, radiation, and light, interchangeably. 


One of the essential aspects of physical chemistry is to recognize the interaction of molecules with the surroundings. With molecular spectroscopy, you can clearly imagine how a polyatomic and diatomic molecule interacts with its surroundings. Further, you can study how any polyatomic or diatomic molecule interacts when studying the wavelength, frequency, energy, wave number, and molecular process. The absorption properties of any polyatomic or diatomic molecule in different regions can also be seen from the electromagnetic spectrum. 


Information of molecules is clearly provided by the electromagnetic spectrum about if there are electronic transitions, rotational transition, or vibrational transition. A molecule or a group of molecules is usually read by seeing the absorption of microwave radiation that provides transitions between energy levels which are rotational. Additionally, when a molecule or group of molecules can absorb infrared radiation, it can provide the transition between the vibrational levels, which is followed by the transition between rotational levels of energy. 


Electromagnetic Radiation

Electromagnetic radiation is simply a form of energy. Waves and particles both describe the behaviour of electromagnetic radiation. The dual models involving wave and particle behaviour give us an informative description of electromagnetic radiation. Although, the exact nature of it still remains unclear since the development of the laws of quantum mechanics at the beginning of the 20th century. 


Properties of Electromagnetic Radiation

  • When the refraction of electromagnetic radiation occurs, it passes from a medium to another, and it can be explained in the best way possible by describing light as a wave. 

  • Properties like absorption and emission can be better explained when treating light as a particle. 

Vibrational Transitions Accompanied by Rotations

While studying spectroscopy, it is important to know how each of the peaks of rotational transitions and vibrational transitions correlates with the molecular processes. In the first place, a molecule’s rotation is possibly affected by the molecule’s vibrational transition. This happens because of a change in bond length which is responsible for rotational transitions that might occur. 


Now that we know the basics of spectroscopy look at some of the basic solved examples that will help you ace your exams. 


Solved Examples

Q1. Among the Options, Which One Can be Considered a Photographic Negative of an Emission Spectrum?

Option 1. Emission spectra

Option 2. Absorption spectra

Option 3. Atomic spectra

Option 4. Spectrum

Answer: Option 2. Absorption spectra


Q2. Choose the Term Which is Used for a Band of Colours.

Option 1. Spectra

Option 2. Spectral analysis

Option 3. Spectrum

Option 4. Dispersion

Answer: Option 3. Spectrum


Q3. Choose the One Which Is the Analysis of Electromagnetic Radiation.

Option 1. Spectroscopy

Option 2. Astronomy

Option 3. Kaleidoscopy

Option 4. Anatomy

Answer: Option 1. Spectroscopy


Q4. in an Atomic Emission Spectrum, Which One of These Lines is Present?

Option 1. Light

Option 2. Bright

Option 3. Brown

Option 4. Black

Answer: Option 2. Bright


Q5. Choose the One Which Has Lower Energy.

Option 1. Initial state energy level

Option 2. Excited-state energy level

Option 3. Ground state energy level

Option 4. None of these

Answer: Option 3. Ground state energy level

FAQs (Frequently Asked Questions)

Q1. How Do You Define Spectroscopy in Simple Terms? 

Answer: The concept of spectroscopy clearly explains the dispersion of light into different component colours. Also, spectroscopy could be termed to measure how much light will be absorbed by any chemical substance. As we have learnt in the beginning, every compound or element has its own unique electromagnetic spectrum. Each of the elements or compounds is responsible for absorbing and dispersing light over various ranges of wavelengths. 


However, spectroscopy is used for different purposes in physics and chemistry. In chemistry, it is used to analyze various compounds and elements. In physics, it helps to understand what makes up the atmosphere of different planets. Students keen on learning about spectroscopy have the opportunity to learn and explore more about quantum mechanics and how it works.

Q2. What are the Applications of Spectroscopy?

Answer: Spectroscopy is an analytical method that helps find the constituents having undiscovered chemical compositions. It also helps in studying and analyzing the structures of different atoms and molecules. In a spectroscopic analysis, you can detect a trace element’s concentration of a few parts per million through an emission spectrum. 


In astronomy, the contribution of spectroscopy is vast. A study of the distant galaxy’s spectral emission lines led to the discovery of the universe’s rapidly and isotropically expanding nature. The study was further based on the observation of the Doppler shift of the spectral lines.