Emission spectra

Emission spectra

1. THE ELECTROMAGNETIC SPECTRUM

   • Electromagnetic waves can travel through space and, depending on the wavelength, also through matter.
   • The velocity of travel c is related to its wavelength λ and its frequency f.
   • Velocity is measured in m s-1, wavelength in m and frequency in s-1
   • The smaller the wavelength and thus the higher the frequency the more energy the wave possesses.
   • c = λ x f
   • Electromagnetic waves have a wide range of wavelengths ranging from low energy radio waves to high energy y-radiation.
   • Visible light occupies a very narrow part of the spectrum.

2. ATOMIC EMISSION SPECTRA

   • White light is made up of all the colours of the spectrum
     • When it is passed through a prism a continuous spectrum of all the colours can be obtained.
   • When energy is supplied to individual elements they emit a spectrum which only contains emissions at particular wavelengths
   • Each element has its own characteristic spectrum known as a line spectrum as it is not continuous.

3. EXPLANATION OF EMISSION SPECTRA

   • When energy is supplied to an atom electrons are excited (gain energy) from their lowest (ground) state to an excited state
   • Electrons can only exist in certain fixed energy levels
   • When electrons drop from a higher level to a lower level they emit energy
   • This energy corresponds to a particular wavelength and shows up as a line in the spectrum.
   • When electrons return to the first level (n = 1) the series of lines occurs in the ultraviolet region as this involves the largest energy change.
   • The visible region spectrum is formed by electrons dropping back to the n = 2 level
   • the first series in the infrared is due to electrons falling to the n = 3 level.
   • The lines in the spectrum converge because the energy levels themselves converge.