Explanation of forming chemical compounds is based on two concepts contradicting each other: one of them stresses similarities of particular parts merging together in a single molecule, and the other shows their opposing properties. During subsequent periods in history, chemists were alternately in favour of one theory and then the other. In reality, parts of molecules are bound together both on the basis of their similarities and opposites. However, in each type of molecule share of these parts is different: from the smallest to absolutely predominant.
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