Transitions Associated with Radiation (College Board AP® Chemistry): Revision Note

Transitions Associated with Microwave Radiation

• Molecules and atoms can absorb light particles (photons)

• When they absorb light, the molecule or atom is promoted to an excited state

• Microwave photons have the lowest energy of the three regions on this page

• When a molecule absorbs a photon of microwave radiation, it undergoes a transition to a higher rotational energy level

  • Therefore, the molecule is promoted to an excited rotational state

• A molecule can rotate about different axes, as the diagram of a water molecule shows

• Absorbing a microwave photon moves the molecule to a higher rotational energy level, so it rotates faster

  • Only photons whose energy matches the gap between two rotational levels are absorbed

Rotational axes in a water molecule

A water molecule can rotate about three different axes

• One of the most used examples regarding excited rotational states is the use of the microwave oven

  • The microwave oven transfer rotational energy to the water molecules in your food

Transitions Associated with Infrared Radiation

• Molecules and atoms can absorb low energy infrared photons

• When they absorb a photon, they are promoted to an excited state

• When a molecule absorbs a photon of infrared radiation, it undergoes a transition to a higher vibrational energy level

  • Therefore, the molecule is promoted to an excited vibrational state

• The modes of vibration of molecules can be analyzed by understanding the covalent bonds as springs

• Like a set of springs, a molecule can vibrate in several different ways (modes)

  • This will depend on the frequency of the photon with infrared radiation that is absorbed

Modes of vibration for the CO₂ molecule

Different modes of vibration in a CO₂ molecule. Each mode has a characteristic frequency of vibration

•  The analytical technique that exploits the changes in vibrations of atoms to identify compounds is called infrared spectroscopy 

Transitions Associated with Ultraviolet/Visible Radiation

• Molecules and atoms can absorb photons in the visible and the ultraviolet region of the spectrum

• When they absorb a photon, they are promoted to an excited state

• The energy of visible and ultraviolet photons is high enough to stimulate electronic transitions

  • Therefore, the electrons are promoted from low energy orbitals to high energy orbitals

  • When electrons return to the ground state, energy is emitted

Absorption and emission diagram

The difference between absorption and emission depends on whether electrons are jumping from lower to higher energy levels or the other way around

• When an electronic transition occurs, it is usually followed by changes in the rotational and vibrational states in a molecule

• The energy-level diagram summarizes which region of the electromagnetic spectrum causes each type of transition

  • Photons with higher energy are needed to reach electronic excited states

  • On the other hand, lower energy photons (infrared and microwave) can be absorbed to reach vibrational and rotational excited states

Summary of the molecular and electronic transitions

Each region of the electromagnetic spectrum excites a different type of transition: microwave → rotational, infrared → vibrational, UV/visible → electronic