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Some recalls of physics: A molecule is a building electrically neutral formed of a limited number of atoms. The atoms of the elements combine in order to acquire a more stable electronic structure. The covalent bond consists of the pooling by two atoms of one or several pairs of electrons, called doublets of connection. In a molecule, each atom in general has the same number of electrons as the rare gas which follows it in periodic classification. Covalency d’un element results then from its place in this classification. The covalent bond is a directed connection. We will use the representation of Lewis. The core and the electrons of the layers intern will be represented by the symbol of the element. The electrons of the external layers will be illustrated by points if they are unmarried or by an indent if they form a doublet and surround the symbol of the element. |
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To obtain water of formula
To balance the formula, one needs two hydrogen atoms for an oxygen atom (see it opposite). |
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Representation of Lewis: the free electrons of the two hydrogen atoms are fixed at the two free atoms of the oxygen atom. |
Calculation of the molar mass of water with M=mass molar
M(water) = 2 M (H) + M (O) = 2 x 1 + 16 = 18 g/mol = 18 g.mol-¹
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, two elements are
used: the hydrogen and the oxygen of respective formula H and O.
