The Octet "Rule"

When elements have more than 10 electrons (two in the first level and eight in the second), the additional electrons are accommodated at higher energy levels beyond the second. Although the third and successive levels can hold more than eight electrons each, the outermost energy level can never hold more than eight electrons (two s and six p electrons), no matter how many total electrons an atom may have. This constraint plays an important part in determining the chemical reactivity of such elements.

In the 1860s, when Dmitri I. Mendeleev in Russia and Julius L. Meyer in Germany began to organize the known elements in a sequence of increasing atomic numbers beginning with hydrogen, they noticed that elements with very similar chemical properties, such as the tendency to react with other specific elements, appear at regular intervals in the list rather than adjacent to each other. Fluorine (atomic number 9) reacts more like the element below it in the periodic table, chlorine (17), for instance, than like an adjacent element with a closer atomic number, oxygen (8). The tendency for chemical properties to recur periodically throughout the sequence of elements is known as the Periodic Law.

We now know that the chemical properties of elements are largely determined by the electronic configuration of their outermost energy level, and this is what accounts for the recurring pattern of the periodic table of the elements. As a rule, elements with the same electron configuration in their outermost energy level (also known as the valence energy level) have similar chemical properties and are aligned vertically in the periodic table. For example, if the orbitals of the valence energy level are filled, as in the case of the so-called noble gas elements [helium (atomic number 2), which has two s electrons, and neon (10) and argon (18), both of which have two s and six p electrons (a total of eight electrons) in their valence energy levels] the element will have very little tendency to react chemically with other atoms and hence will be extremely stable. In general, when atoms have their valence energy levels complete, with eight electrons (two s and six p), they are relatively inert.

The electrons in the outermost energy level are often called the valence electrons.  Atoms are in a particularly stable configuration when the s and p orbitals of the valence energy level are filled, that is, when each atom has eight electrons in its valence level,* sometimes referred to as the octet rule. Depending on their electronic configuration, atoms may react with other atoms and give up, take, or share their valence electrons, forming chemical bonds between the atoms.