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 aroundthe 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 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. 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.

---