What is the octet rule?
The octet rule denotes the tendency that the elements in the S and P blocks of the periodic system preferably will have 8 electrons in the outer shell. In this way, the element will look like a noble gas that is not reactive. The octet rule can be used both in connection with ions and covalent bonds . In relation to ions, one can basically expect that an element will form the ion which has 8 electrons in the outer shell. For example, fluorine (F) with 7 electrons will ultimately absorb an electron and become fluoride ion F–.
In relation to covalent bonds; the elements covered by the octet rule will always form bonds so that they are surrounded by just 8 electrons. By dividing one or more pairs of electrons, all the constituents of a molecule can achieve 8 electrons in the outer shell. As an example, the carbon dioxide electronics formula CO2 is shown below.
Here it is seen that both the 6 electron oxygen atoms and the 4 electron carbon atom in the outer shell must all be surrounded by 8 electrons, and thus the molecule meets the octet rule.
The lightest 5 elements are not covered by the octet rule, as they only have 1 to 5 electrons in total. Therefore they can not easily obtain 8 electrons. Instead, these elements would like to have two electrons in the outer shell. This is called the dublet rule. The main effect of the dublet rule is to understand that the hydrogen atom (H) only needs 2 electrons in total to resemble a noble gas, namely helium (He). The water molecule below fulfills both the dublet rule and the octet rule.