EMISSION AND ABSORPTION SPECTRA

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Spectra

A spectrum is the pattern formed when a beam of light (EM – radiation) is broken up into its component frequencies A “Spectroscope” is an instrument, which utilizes either a prism or a diffraction grating to provide a spectrum, and then brings the spectrum into focus 2 types of spectra: 1) Absorption spectrum - A continuous spectrum where certain frequencies are missing - An atomic absorption spectrum is formed when light passes through a cold diluted gas. The gaseous atoms absorb photons of specific frequency. The light that passes through the gas, produces a spectrum that has lines missing that correspond to the particular absorbed frequencies - Each element produces a characteristic absorption spectrum with the missing frequencies corresponding to the photons of light that have energies equal to the different energy levels in the atoms (of the element) 2) Emission spectrum - A characteristic spectrum produced by a light source 2 main types of emission spectra… a) Continuous emission spectrum - A spectrum in which one colour flows into another without any well defined boundary and no absence of frequencies

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b) Line emission spectrum - When an electric current is passed through a gas, the gas begins to emit - A gas discharge tube is a tube that contains a gas that has a large current passing through it - The spectrum that is observed (through a diffraction grating or spectroscope) is called an emission spectrum and shows separate bright lines of different colours (each element has its own unique set of lines) - These lines are a result of the fact that atoms can absorb or emit energy - The lines that appear in the spectrum correspond to fixed energies of light that are emitted by the atoms in the gas. The electrons in the atoms of the gas are excited by heating or electrical charge to higher energy levels within the atom (excited state). The electrons spontaneously decay back to the lower levels (ground state) as they are more stable in these states. - When the electron moves back down, the difference in energy is given off as a photon with a very specific frequency (E = hf).

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Emission spectrum of hydrogen Consider the diagram depicting the energy levels of a hydrogen atom…

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The levels are labelled n = 1 for the ground state, n = 2 for the first excited state, etc.

The emission lines that result when an electron drops to the n = 2 level produce a spectrum known as the Balmer – series. These lines all fall in the visible spectrum There are lines however that don’t show up in the visible range, but exist in the infra red and ultraviolet range The series that appears in the ultra violet region (transitions to ground state, n =1) is known as the Lyman series The series that appears in the infrared region (transitions to the n =3 state,) is known as the Paschen series

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The light from an activated gas is therefore not a continuous spectrum, but rather a number of discreet lines with dark spaces between (absence of light) Since the emitted photons consist only of certain energies (rather than a continuum of energies), it implies that the internal energy of the atom will only increase or decrease in particular “steps” or “levels” The energy released can be calculated as follows… E = photon energy = E n – E m = hf Where: n > m Calculation: a) The wavelength of the blue line in a hydrogen spectrum is approximately 415nm. Calculate the energy of a photon of blue light.

b) If the above photon was the result of an electron moving from an energy level of 5,25x10 -19 J, calculate the energy of the other level involved.