<text><span class="style10">mall Molecules (1 of 3)</span><span class="style7">Although the Earth's atmosphere consists almost entirely of two gases - nitrogen and oxygen - a number of other gases are present at low concentration, together with varying amounts of water vapor. With the exception of the noble gases, most other components of air form part of natural cycles, each remaining in the atmosphere only for a limited time. Not only are these gases of major importance in relation to industrial processes that dominate economies throughout the world, but cyclical processes involving water, oxygen, carbon dioxide and nitrogen - together with solar radiation - are essential to plant and animal life. Current interest in various atmospheric gases centers on the possible global effects of changes in their atmospheric concentration due to human activities. Increase in carbon dioxide may upset the heat balance at the Earth's surface, while the use of chlorofluorocarbons (CFCs) might result in depletion of the ozone layer, thereby allowing destructive high-energy solar radiation to reach the Earth's surface.Although these small molecules are simple in the sense that they are composed of few atoms, their structures and - for those with three or more atoms - their shapes vary. In most cases, their atoms are held together in the molecule by two, four or six electrons, resulting in single, double or triple covalent bonds. Three of these molecules (nitric oxide, nitrogen dioxide and oxygen) are paramagnetic - i.e. attracted to a magnet, like iron - because of the number or arrangement of their electrons.</span></text>
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<text><span class="style10">ry ice vaporizing.</span><span class="style7"> A colorless gas at room temperature, carbon dioxide becomes a liquid under compression. If the liquid is allowed to expand rapidly to atmospheric pressure, it becomes cooler and freezes to dry ice ΓÇö a white, snowlike solid. Dry ice sublimes (passes directly into vapor without melting) at ΓÇö78.5 ┬░C (ΓÇö109.3 ┬░F); it is used to produce stage fog and as a refrigerant.</span></text>
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<text>ΓÇó WHAT IS CHEMISTRY?ΓÇó CHEMICAL BONDSΓÇó THE EARTH'S STRUCTURE AND ATMOSPHEREΓÇó THE HYDROLOGICAL CYCLEΓÇó WEATHERΓÇó PLANT PHYSIOLOGYΓÇó THE BIOSPHEREΓÇó THREATS TO THE ENVIRONMENT</text>
