1.3.3

Absorption & Emission Spectra

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Electromagnetic Radiation

Electromagnetic radiation is a continuous spectrum of waves with different energies.

The electromagnetic spectrum

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

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

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

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

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

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.

Emission Spectra

Each element has a characteristic emission spectrum depending on its electron configuration.

Emission spectra

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

Balmer series

  • Electrons that fall back to n = 2 correspond to transitions that release visible light and make up the Balmer series.
Significance

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.
Jump to other topics
1

Structure - Models of the Particulate of Matter

2

Structure - Models of Bonding & Structure

3

Structure - Classification of Matter

3.1

The Periodic Table: Classification of Elements

3.2

Periodic Trends

3.3

Group 1 Alkali Metals

3.4

Halogens

3.5

Noble gases, group 18

3.6

Functional Groups: Classification of Organic

3.7

Functional Group Chemistry

3.8

Alkanes

3.9

Alcohols

3.10

Halogenoalkanes

4

Reactivity - What Drives Chemical Reaction?

5

Reactivity - How Much, How Fast & How Far?

6

Reactivity - The Mechanisms of Chemical Change

7

Measurement, Data Processing & Analysis

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