OCR AS Chemistry F332: Radiation and Matter

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Radiation and Matter
Electromagnetic radiation can interact with matter, transferring energy to the
chemicals involved.
The chemicals absorb energy and this can make changes happen in the chemicals.
What changes occur depend on:
The chemical involved
The amount of energy involved
Molecules are doing energetic things all the time ­ they move around, they rotate and
the bonds in the molecule vibrate.
The electrons in the molecule have energy too, and they can move between the
different electronic energy levels.
A molecule has energy associated with several different aspects of its
behaviour including:
Energy associated with translation
Energy associated with rotation
Energy associated with vibration
Energy associated with electrons
These different kinds of energetic activities involve different amounts of energy ­ for
example, making the bonds in a molecule usually involves more energy than making the
molecule as a whole rotate.
The energy needed increases in the general order:
Electronic energy
Vibrational energy increasing energy
Rotational energy
Translational energy

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Electrons can occupy definite energy levels.
The electronic energy of an atom or molecule changes when an electron moves from
one level to another.
We say that electronic energy is quantised, with fixed levels.
All these other types of energy (translational, rotational and vibrational) are quantised too.
For example:
An HCl molecule can occupy only certain fixed levels of vibrational energy.…read more

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Different Energy Changes for Different Parts
of the Spectrum
The spacing between different vibrational energy levels corresponds to the infrared part of
the spectrum.
We sense infrared radiation as heat. You can sense infrared radiation when you feel the
warmth radiated by a fire.
The radiation makes bonds in the chemicals in your skin vibrate more energetically.
The energy in the radiation has been converted into kinetic energy and this is why you
feel warmer.…read more

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Electronic Changes and Ultraviolet Radiation
Electrons occupy definite energy levels in atoms.
The gaps between these electronic energy levels correspond to the energy of visible and
ultraviolet light. So electrons jump to higher energy levels when an atom absorbs photons of
visible or ultraviolet radiation.
The same kind of thing happens with molecules. The electrons in a molecule, such as Cl 2,
occupy definite energy levels. This is true of the electrons that bond the atoms together and
also of the non-bonding electrons.…read more


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