Fully filled outermost shell:

The arrangement in that the outermost shell is either fully filled (as along with He and Ne) or holds eight electrons (as along with Ne, Ar, Kr, Xe, Rn) is known as the inert gas configuration. An inert gas configuration is exceptionally stable energetically since these inert gases are the least reactive of all the parts.

The first components in the periodic table, hydrogen, do not have properties which satisfactorily place it in any group.  The Hydrogen has two unique categories: (a) the highest energy shell of a hydrogen atom could hold just two electrons, in compare to all others (except helium) which could hold eight or more; and (b) while hydrogen loses its electron, an ion formed, H⁺, is a bare nucleus. The hydrogen ion is extremely small within comparison with a positive ion of any other element that must still have a few electrons surrounding the nucleus. Hydrogen could either gain or lose an electron.   It has a few properties same to Group IA elements, and a few same to Group VIIA elements.

The number of electrons within the outer, or valence, shell determines the associative activity of the element.  An element is arranged in the periodic table so in which elements of the similar group have the similar number of electrons within the outer shell (except for the Transition Groups).  An arrangement of electrons in the outer shell explains why a few elements are chemically extremely active, a few are not extremely active, and others are inert. In common, the fewer electrons an element have to be losing, gain, or share to reach a stable shell structure, the more chemically active the element is. A likelihood of elements form compounds is strongly influenced through this valence shell and on the stability of the resulting molecule.  The more stable the molecules are, a more likely those molecules are to form.