The octet rule does not apply to transition and inner transition metals and to the first for or five elements in the periodic table.
Cl-
Calcium.
For an element to be stable, it must have a full outer shell (valence shell) of electrons. An element with a full outer shell of electrons will act like its nearest noble gas and be stable. Most elements require 8 valence electrons to be stable, however H needs 2.
Valence Electrons are the electrons that are located furthest away from the atom itself in the outermost electron shell. They are located on the last energy level also known as the valence level.
Most elements need 8 electrons in their valence shells.
stable electronic configuration
Cl-
Alkali metals, have only one valence electron so they will "want" to lose it to achieve a stable electron arrangement.
8 valence electrons
Noble gases are inert because they contain a "stable octet" of electrons in the outermost shell of the atom (valence electrons). This means that the noble gases do not need to react to lose or gain electrons in order to become stable, since scientists have determined that 8 valence electrons is a stable electron configuration.
All noble gases have completely filled orbitals. Helium has two valence electrons and its s-orbital is full and is stable. Other noble gases have completely fill p-orbitals as the valence orbitals.
Valence electrons are the amount of electrons in the outermost electron shell. 8 valence electrons fill the outer shell making it completely stable.
An oxygen atom has 6 electrons in its outer shell, therefore it needs two more to have a stable configuration of 8 electrons. That's why the valence is two.
A stable atom has 8 electrons in its outer most valence shell. A simple way to remember this is that all atoms want to be like the noble gases which all have 8 electrons (except helium but the reason is complicated and not necessary here)
Neutral Carbon atoms contain 6 electrons and 6 protons 2 electrons are found in the 1st electron ring and 4 in the outer ring to reach a stable electron (8 in the outershell) arrangement carbon requires 4 covalent bonds to be formed
An atom requires 8 valence electrons to be chemically stable. The elements with 8 valence electrons are the Noble Gases, and they are both stable and largely unreactive.
The noble gases - group 18 on most newer periodic tables - are the most stable. They have a completely filled valence shell of electrons.