<text><span class="style10">hat is Chemistry? (4 of 6)Elements and molecules</span><span class="style7">The structure of atoms serves as a convenient starting point for discussing chemical phenomena. In chemical processes, the nuclei of atoms remain unchanged - shattering at once the al-chemist's dream of transmuting elements. The great variety of known chemical compounds results from the different ways in which the electrons of atoms are able to interact either with atoms of the same kind or with atoms of a different kind. In an </span><span class="style26">element</span><span class="style7">, all the atoms are of the same kind, but the varying strengths of the interactions between the electrons in different types of atom means that elements have very different properties. For example, helium melts at -272 deg C (-458 deg F), whereas carbon in the form of diamond has a melting point of 3500 deg C (6332 deg F). This ability of electrons to interact between atoms is known as </span><span class="style26">chemical bonding</span><span class="style7">.The elements nitrogen, oxygen, fluorine and chlorine form strong bonds, with two identical atoms linked together. They therefore exist at room temperature as gases, with pairs of linked atoms moving chaotically in space. Two or more atoms linked in this fashion are described as </span><span class="style26">molecules</span><span class="style7">, and a shorthand notation is used to describe their chemical identity. The atomic symbol for the element is used in conjunction with the number of atoms present to define the </span><span class="style26">chemical formula</span><span class="style7"> of the molecule. The elements described above are therefore designated, respectively, by the formulae N2, O2, F2 and Cl2.Other familiar elements, such as sulfur and phosphorus, form additional bonds to like atoms, and their formulae reflect this fact. Thus sulfur forms a ring of eight atoms and is described by the formula S8. As the number of atoms in the fundamental unit increases, the element is no longer a gas but becomes a solid with a low melting point; thus sulfur can be extracted from the Earth as a molten fluid.Most elements do not form discrete molecular entities such as those described above, but have structures that are held together by chemical bonds in all directions. Most of the 109 known elements are metals, such as iron and copper, and have </span><span class="style26">infinite structures</span><span class="style7"> of this kind. Such elements can no longer be given distinct molecular formulae and are therefore represented by the element symbol alone; thus iron, for example, is represented simply as Fe. For a more detailed discussion of the properties of metals.</span></text>
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<text>Molecules such as bromoacetone react with water on the surface of the eye to produce acids that irritate the eye and cause tears to flow. Tear gas is a variant of this molecule.</text>
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<text>ΓÇó CHEMISTRYΓÇó OIL AND GASΓÇó IRON AND STEELΓÇó RUBBER AND PLASTICSΓÇó CHEMICALS AND BIO TECHNOLOGY</text>
