This table is called periodic because all of the atoms found in the same column (from 1 to 18), exhibit similar chemical properties.

For example, in column 1, Lithium Li and Sodium Na have similar chemical behaviours.

Why?
Because the number of electrons in their outermost orbits are identical. Li and Na each have a single electron circulating in their outermost orbits: Thus it'll be this external electron which will be responsible for chemical reactions with other atoms. Hence the same chemical properties and their chemical grouping.

Atoms then are grouped in families:
 

The family of rare gases exhibit a particular characteristic: Helium, Neon, Argon etc are very stable atoms because they possess an external layer of electrons which is complete. This layer completely full of electrons is very stable and therefore the atom neither gives up nor accepts other electrons.Chemical interaction between atoms are therefore nearly impossible for the rare gases.

This unmasks a golden rule of chemistry: All atoms tend to complete their external layer of electrons, this by capturing or giving up electrons to other atoms.

• Technetium 43
• Samarium 62
• all elements starting from Polonium where Z is greater than or equal to 84.