Periodic Law and the Periodic Table of Elements

In chemistry, the generalization that there is a recurring pattern
in the properties of the elements when they are arranged in order
of increasing atomic number - i.e. the total number of protons in
the nucleus of atoms.  The periods (horizontal rows) of the
periodic table illustrate these relationships.

The elements, arranged in order of their increasing atomic numbers,
are divided into seven periods reflecting periodicities in
electronic structure and in chemical and physical properties. 
There are three short periods of 2, 8, and 8 elements; three long
periods of 18, 18 and 32 elements, all of which are known; and a
further long period predicted to contain 32 elements with atomic
numbers from 87 to 118.

The progression of elements along a period represents a systematic
occupation by electrons of orbitals, or electron shells.  A period
is completed when all the electron orbitals within a given energy
level are filled.  