State and describe the octet rule.                     

A chemical rule of thumb which redirects the observation that the bonding in elements tends to follow in the way where the valence shell of each atom should hold eight electrons and have the electronic configuration same as the noble gas. It was observed that due to the electronic configuration, noble gas are unreactive. Neon has 1s22s22p6 electronic electron configuration, and the outer shell is full and more electrons cannot incorporate into the valence shell. All the other noble gasses have same valence shell configuration of electron though having different inner-shell numbers of the electron. This was observed by Gilbert Lewis, an American chemist, and this observation lead to the explanation of molecules and ions that other elements forms. Especially octet rule applies to oxygen, nitrogen, carbon even to the metals like magnesium and sodium and to the halogens. Lewis electron dot diagram can be applied to count the electrons in the valence shell. If two atoms share the electrons in a covalent bond, it is counted twice, one for each atom. Such as in carbon dioxide where each oxygen atom shares it is four electrons with a central carbon atom, two from the atom of carbon and the other two from the atom of oxygen. These shared electrons are counted in both the octet of oxygen and carbon.

How can it be used to predict outcomes of chemical reactions?

To satisfy the octet rule, there are two ways that an atom can follow. One way is to share its valence electron with the other atoms. The second is to transfer its electron in valence shell to one atom to other. To attain octet metals lose all of its electrons from valence shell and thus leads to the lowest principal energy level. Non-metals atom gain electron to fulfil the octet in outermost principal energy level. The number of bonds an element forms in a compound can be predicted by the Lewis dot symbols. To predict the necessary arrangement of bonds the Lewis electron-dot symbols can be employed, it consists the elements chemical symbol surrounded by dots which represents the valence shell electrons, placed above, below and to the left and right in pairs of the symbol. The unpaired dots are used by Lewis to predict that an element in its compound will form how many numbers of bonds. It also explains the formation of three bonds by nitrogen in a compound where nitrogen shares the unpaired three valence electrons. Boron, in the same manner, shares the three valence electron to form three bonds in a compound. Same in case of carbon, the four unpaired electrons in the valence shell are shared to form the compound with four bonds to attain the neutral charge or octet.

In the case of sodium chloride NaCl where chlorine has seven electrons, and sodium has one in valence shell; thus to attain octet sodium will lose one electron and chlorine will gain one Na+ and Cl-. However, if chlorine had to lose seven electrons, their charges will be much higher, resulting in less stable compound Na7- and Cl7+. The structure shows that elements in the 2nd period and beyond tend to lose, gain or share their electrons to attain eight electrons in their valence shell in their formed compound. The only Hydrogen who has two electrons in the valence shell, do not follow the octet rule.