What would you say about a contractor who erected a building and after putting the roof on, asked the designers to calculate whether everything had been done right?

It sounds like an episode from �Through the Looking Glass,� doesn�t it?

Nevertheless that was just what happened to the Periodic System of Elements. The Big House was first built and the elements lodged, each in a flat of its own. Chemists made Mendeleyev�s Table their tool. But why the properties of the elements repeat themselves periodically they could not tell for a long time.

The explanation was supplied by the physicists. They calculated the structure of the Periodic System building for strength, and their findings were remarkable. They found it to have been built absolutely right in accordance with all the laws of �chemical mechanics:� And so we cannot but admire Mendeleyev�s truly great intuition and profound knowledge of chemistry.

Physicists began by studying the structure of the atom in detail. The heart of the atom is its nucleus. Around it revolve electrons, the number of which equals the number of positive charges on the nucleus. Hydrogen has one electron, potassium has twelve, uranium has ninety-two. How do they revolve? Chaotically, like a swarm of moths fluttering around an electric light bulb, or in some definite order?

To answer this question the scientists had to resort to new physical theories and to develop new mathematical methods. And here is what they found: electrons revolve around the nucleus on definite orbits, like the planets around the Sun. �How many electrons are there on each orbit? Any number at all or a limited number?� asked the chemists.

�A strictly limited number!� replied the physicists. �All electron shells possess a finite capacity.

Physicists have their own symbols for electron shells. They use letters They use letters K, L, M, N, O, P, and Q. These letters denote the shells in order of their remoteness from the nucleus.

In collaboration with mathematicians, the physicists drew up a detailed scheme indicating how many electrons each orbit contains.

The K-shell can have 2 electrons and no more. The first of them appears in the hydrogen atom, and the second in the helium atom. That is why the first period of the Mendeleyev Table consists of only two elements.

The L-shell can accommodate many more, namely, 8 electrons. We find the first electron belonging to this shell in the lithium atom and the last in the neon atom. The elements from lithium to neon form the second period of Mendeleyev�s System.

And how many electrons are there in the subsequent shells? The M-shell accommodates 18, the N-shell 32, the O-shell 50, the P-shell 72, etc.

If the outermost electron shells of two elements are identically arranged, the properties of the elements are similar. For instance, lithium and sodium, each contains a single electron in its outer shell. And this accounts for their being in the same group of the Periodic System, namely, in the first group. Note that the group number equals the number of valence electrons in the atoms of the elements in the group.

And now the conclusion: outer electron shells of identical structure recur periodically. And that is why the properties of the elements also recur periodically.

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