This article presents a personal answer to the question “What is chemistry?”, set out in terms of six propositions. These cover “pure” and “applied” chemistry, different levels of description, and the broader context of chemistry.

A modern version of Lewis’s theory of valency is presented. This takes account of the results of quantum–mechanical calculations on molecules. Topics covered are polar covalent bonds, hypervalency, coordinate bonds, nonintegral bonds, oxo-anions, variable valency among transition elements, and nonclassical compounds. A distinction is drawn between the valence shell of an atom and the Lewis shell. The concept of a fractional bond pair is presented.

The spiral form of the Periodic Law is proposed as its fundamental graphic representation. This idea is based on the fact that the spiral is the most appropriate form in description transitions from simple to complicated. The spiral is easily obtained from the linear succession of the elements when they are ranged by growing nuclear charge. The spiral can be simply transformed into many other graphic representations, including tables. This paper suggests the conception of the autonomy of blocks. This autonomy (...) is clearly seen in the variation of the outer electronic shells, in the width and the height of the blocks, as well as the number and properties of elements included therein. The regularities in the changes of element properties are pronounced in certain blocks but actually absent in others. The blocks can be permuted to obtain full-fledged versions of Periodic Table. The new stage of the verification of the blocks autonomy is the use of the total number of the differentiating electrons as an independent variable in describing the properties of the elements and their compounds. Consideration of individual blocks made it possible to deduce the periodic equations valid for all the elements within each block. Formulation of the Periodic Law is advanced: while describing characteristic determining the properties of the elements, the nuclear charge is replaced by the total number of differentiating electrons. Two modifications of the conventional Periodic Table are obtained by minimal changes. The first one shows the secondary and the additional periodicites. Another table shows the total number of differentiating electrons for each element. (shrink)