Topic 3.11 Notes – Spectroscopy and the Electromagnetic Spectrum

1. What Electromagnetic Radiation Is

Electromagnetic (EM) radiation behaves as both a wave and a particle. You need both ideas to understand spectroscopy.

Wave behavior

Two properties describe a wave:

• Wavelength (λ)
  Distance between repeating points like peak-to-peak.

• Frequency (ν)
  Number of waves passing a point per second (units = Hz or s⁻¹).

This diagram shows how wavelength is measured along a wave (and also labels amplitude for reference):

Basic wave properties: wavelength and amplitude

These two are linked by:

$$c = \lambda \nu$$

• $c = 3.00 \times 10^{8} \text{ m/s}$ (speed of light)
• If $\lambda$ increases, $\nu$ decreases.
• Short wavelength → high frequency.

Particle behavior

Light also comes in packets called photons.

Each photon has energy:

$$E = h\nu$$

• $h = 6.626 \times 10^{-34} \text{ J}\cdot\text{s}$
• Higher frequency → higher energy photon.

Combine the equations and you see:

• Shorter $\lambda$ → higher $\nu$ → higher $E$

That energy is what drives molecular and electronic changes.

The key idea

Atoms and molecules have quantized energy levels. They can only absorb or emit energy in specific amounts.

A photon will only be absorbed if:

$$E_{\text{photon}} = \Delta E_{\text{levels}}$$

No exact match, no transition. That’s a favorite conceptual test question.

2. The Regions of the Electromagnetic Spectrum

Ordered from highest energy / shortest wavelength to lowest energy / longest wavelength:

1. Gamma rays
2. X‑rays
3. Ultraviolet (UV)
4. Visible
5. Infrared (IR)
6. Microwaves
7. Radio waves

Trends to lock in

• Moving from gamma rays toward radio waves (high energy to low):
  • Wavelength increases
  • Frequency decreases
  • Energy decreases

Visible light is only a tiny slice of the spectrum, about 400-700 nm:

• Violet ≈ shortest λ (highest energy in visible)
• Red ≈ longest λ (lowest energy in visible)

For AP Chem, the three most important regions are:

• Microwaves
• Infrared
• UV/Visible

Each corresponds to a different type of transition.

3. Types of Energy Transitions and Their Spectral Regions

Energy spacings in molecules are not all the same size. Smaller gaps require lower-energy radiation.

a. Rotational Transitions - Microwaves

• Involve whole molecules rotating
• Very small energy differences
• Observed mostly in gas phase

Microwave photons provide just enough energy to move between rotational levels.

Small gap → low energy → long wavelength.

b. Vibrational Transitions - Infrared

• Involve bond vibrations
  • Stretching
  • Bending
• Bonds behave like tiny springs.

Infrared radiation excites molecules to higher vibrational levels.

Important connection for class:

• Different bonds absorb characteristic IR frequencies.
• That’s how IR spectroscopy identifies functional groups.

Vibrational energy gaps are larger than rotational ones, so IR has more energy than microwaves.

c. Electronic Transitions - Ultraviolet and Visible

• Involve electrons moving between energy levels
• Much larger energy gaps
• Usually valence electrons

UV/visible photons promote electrons to higher electronic states.

Why substances have color:

• They absorb certain visible wavelengths.
• The remaining transmitted or reflected light is what you see.

Electronic transitions require the most energy of the three.

4. Absorption and Emission of Photons

Two processes matter:

Absorption

• Photon is absorbed.
• System moves to a higher energy level.
• Photon energy equals the energy gap.

Emission

• Excited state relaxes.
• Photon is released.
• Emitted photon energy equals the energy difference.

Here’s the energy picture. Focus on the upward arrow for absorption and the downward arrow for emission, with $\Delta E = h\nu$ in both cases.

Energy-level diagram for absorption and emission

On exams, they love asking which region matches a given transition type. Think in terms of energy gap size, not memorized trivia.

Small gap → microwave → rotation
Medium gap → IR → vibration
Large gap → UV/visible → electronic

5. Big Picture Relationship

Memorize this order of increasing energy:

Rotation < Vibration < Electronic

Which matches:

Microwave < Infrared < UV/Visible

If you understand that the spectrum maps directly onto the size of the energy gap, everything in this topic falls into place.