The columns,
aslo caled a group, in chemical periodic system represent the number of valent
electrons.
Valent electrons
are electrons located in outermost electron shells, and they participating in forming
of chemical bond. The old notaion of columns were greek numbers from I (1) to
VII (7), representing the number of valent electrons, with exception of 0 (zero) for
noble gases which have 8 valent electron. The modern numbering of a groups is
recommended by the International Union of Pure and Applied Chemisty (IUPAC) starting
from 1 to 18.
