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The problem of the peculiar character of chemical laws and theories is a central topic in philosophy of chemistry. One of the most characteristic and, at the same time, most puzzling examples in discussions on chemical laws and theories is Mendeleev's periodic law. This law seems to be essentially different in its nature from the exact laws of classical physics, the latter being usually regarded as a paradigm of science by philosophers. In this paper the main arguments concerning the peculiar character of chemical laws and theories are examined. The laws of chemistry are natural laws to the same extent as are the laws of physics. The law discovered by Mendeleev is a normal law of nature. It is not a law of physics, nevertheless, it is exact in the same philosophical sense as are the laws of physics. The periodic system of chemical elements was established by constructing an idealized system of idealized elements. The fundamental idealization substantiated by experimental chemistry was the chemical element as a place in the periodic system.