Electromagnetic radiation can interact with matter, transferring energy to the chemicals involved. They absorb energy, and this can make changes happen to them. The changes that can occur depend on the chemical involved and the amount of energy. A molecule has energy associated with different aspects of its behaviour:
These different energy activities involve different amounts of energy. The energy needed increases from 1 to 4.
We have seen before that electrons can occupy definite energy levels. The electronic energy of an atom or molecule changes when an electron moves from one level to another. Electronic energy is quantised. In fact all other types of energy are quantised. A molecule of HCl can occupy certain fixed levels of vibrational energy. When its vibrational energy changes, it moves to a new, fixed energy level, in a similar way to electrons. This corresponds to a photon of infra-red radiation.
The spacing between vibrational energy levels corresponds to the infra-red part of the spectrum.
Changes in rotational energy correspond to a lower energy, hence lower frequency, in the microwave region.
Spacings between translational energy levels are even smaller and can be treated as being continuous.
Electronic changes require most energy and usually occur in the visible and ultra-violet part of the spectrum.
|Change occurring||Size of energy change /J||Type of radiation absorbed|
|Rotational||1 x 10-22 to 1 x 10-20||Microwave|
|Vibrational||1 x 10-20 to 1 x 10-19||Infra-red|
|Electronic||1 x 10-19 to 1 x 10-16-||Visible & Ultra-violet|
The particular value of the energy change depends on the substance involved. Different substances have different structures, and this gives them different energy levels. A C - F bond is stronger than a C - Br bond, so more energy is needed to make it vibrate.
Electrons in atoms occupy definite energy levels. Electrons in molecules, e.g. Cl2, similarly occupy definite energy levels. When the molecule absorbs visible or uv radiation, one of three things can happen, depending on the amount of energy involved:
The Sun radiates a wide spectrum of energy, part of which corresponds to that required to break chemical bonds. It can break bonds in molecules such as DNA which can cause damage to genes and lead to skin cancer. Even brief exposure to the Sun may cause skin to become burnt.
The most damaging part of the Sun's spectrum is the higher frequency, higher energy per photon ultra-violet region. Glass in windows absorb ultra-violet radiation. Chemists have developed chemicals, which do a similar job that are called sunscreens. The best sunscreen of all, however, is the atmosphere.
Certain atmospheric gases absorb uv radiation. The best of these is ozone which absorbs uv radiation in the region 10.1 x 1014Hz - 14.0 x 1014Hz in the stratosphere. There is no life in the stratosphere; molecules are broken down to form radicals. Above the stratosphere the radiation is so powerful that it ionises atoms, molecules and radicals. Therefore, this region is called the ionosphere.
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