Spectroscopy and the Electromagnetic Spectrum

Learning Objective 3.11.A Explain the relationship between a region of the electromagnetic spectrum and the types of molecular or electronic transitions associated with that region.

Quick Notes

Electromagnetic radiation consists of photons (energy) that travel in waves of differing energies
Photons can be absorbed by matter in different ways, causing atoms or molecules to reach an ‘excited’ state
Substances can be analysed based on the absorption of energy to reach ‘excited states’ in a process called spectroscopy
Microwave radiation causes rotational transitions in molecules
Infrared (IR) radiation causes vibrational transitions in molecules
Ultraviolet (UV) / Visible radiation causes electronic transitions in atoms or molecules

Full Notes

Electromagnetic radiation consists of photons – packets of energy – that travel in waves. The electromagnetic spectrum includes all types of radiation, from low-energy radio waves to high-energy gamma rays. Different types of radiation interact with matter in different ways depending on their energy.

AP Chemistry diagram showing the electromagnetic spectrum with wavelength scale and highlighted UV, visible, IR and microwave regions as energy increases.

When electromagnetic radiation interacts with atoms or molecules, it can be absorbed, causing the particles to transition from their lowest energy state (ground state) to a higher energy state (excited state).

Different types of radiation cause different transitions as they contain different energies.

AP Chemistry energy-level diagram showing ground, rotational, vibrational, and electronic excited states with microwave, IR, and UV photons causing the corresponding transitions.

Microwave Radiation

AP Chemistry spectrum highlight of the microwave region on the electromagnetic spectrum.

Has relatively low energy and causes rotational transitions in polar molecules. This is where energy is absorbed to ‘rotate’ bonding in the molecule to go from a lower energy (ground) state to an excited state.

AP Chemistry illustration of a rotational transition where microwave radiation increases rotation about a molecular bond axis.

Molecules absorb microwave radiation and begin to rotate more quickly
Commonly used in rotational spectroscopy and for studying gases like HCl or CO

Infrared (IR) Radiation

AP Chemistry spectrum highlight of the infrared region on the electromagnetic spectrum.

Has moderate energy and causes vibrational transitions in molecules. Bonds in molecules vibrate more intensely (stretching and bending) when IR radiation is absorbed.

AP Chemistry diagram showing stretching and bending vibrational modes induced by absorption of IR radiation.

Used in IR spectroscopy to identify functional groups in organic compounds
Different bonds absorb at different IR frequencies

Ultraviolet (UV) and Visible Radiation

AP Chemistry spectrum highlight of the ultraviolet and visible regions on the electromagnetic spectrum.

Higher energy radiation that causes electronic transitions: electrons are excited from one energy level to another.

AP Chemistry diagram of electronic transitions between energy levels associated with UV/visible radiation.

UV-visible spectroscopy is used to study the electronic structure of atoms and molecules
Especially useful for analyzing compounds with conjugated systems or transition metals

Summary Table

Matt’s exam tip

You don’t need to memorize specific wavelengths, but you should understand the relative energy of each type of radiation and what type of molecular motion or electronic change it causes.

Summary

Different parts of the electromagnetic spectrum cause different types of transitions in molecules:

Microwave radiation → rotational transitions
Infrared radiation → vibrational transitions
UV/Visible radiation → electronic transitions

Spectroscopy techniques take advantage of these interactions to study the structure and behavior of atoms and molecules. Recognizing the type of radiation and its associated transition is key to interpreting spectroscopic data in chemistry.