Absorption & Emission Spectra
Chemistry: IB Diploma Higher Level, topic 1.3.3
Electromagnetic Radiation
Electromagnetic radiation is a continuous spectrum of waves with different energies.
The electromagnetic spectrum
- As the energy of waves changes across the electromagnetic spectrum so do both the frequency (𝜈) and wavelength (λ).
- Visible light makes up a small part of this spectrum.
Frequency and wavelength
- The frequency (𝜈) and wavelength (λ).of a wave are related by the following formula:
- c = 𝜈λ
- c is the speed of light.
Wave energy
- Frequency (𝜈) is multiplied by Planck’s constant (h) to give the energy (E) of the wave:
- E = h𝜈
- h = 6.63 × 10−34 Js
Absorption Spectra
Each element has a characteristic absorption spectrum depending on its electronic configuration.
Electron excitation
- When electromagnetic radiation is passed through a sample of gaseous atoms, specific energies of radiation are absorbed and cause electrons to be promoted (excited) to higher energy levels.
Absorption spectrum
- This produces an absorption spectrum and the frequencies of radiation that correspond to the energy gap for electrons to be excited are missing from the continuous spectrum.
Absorption spectra are unique
- The energy of an absorbed photon is the same as the energy gap for the electron excitation and this can be calculated from its frequency by using the following formula:
- ΔEelectron = h𝜈
- Each element has its own characteristic absorption spectrum as each element has a unique electron configuration.
Absorption spectra are unique
Emission Spectra
Each element has a characteristic emission spectrum depending on its electron configuration.
Emission spectra
- After excitation, electrons can then relax back to lower energy levels to produce an emission spectrum.
- The emission lines occur at the same frequencies as the absorption lines in the absorption spectrum as the energy gaps for electrons to move are the same.
Balmer series
- Electrons that fall back to n = 2 correspond to transitions that release visible light and make up the Balmer series.
Significance
- The line emission spectrum gives us evidence for the Bohr model of an atom.
- The emission spectrum of hydrogen provides evidence for the existence of electrons in discrete energy levels.
- At high energy, lines start to become closer together and are said to converge. Energy levels converge at n = ∞ and when an electron reaches this level the atom has been ionised.
Test yourself on Absorption & Emission Spectra
What is the symbol for wavelength?
What is the correct equation for the energy of a wave?
What is the correct equation for calculating the energy gap for electron excitation?
Which of these statements is correct?
What is the name given to the series of electron transitions that release visible light?