Why there are exceptions to octet rule?

  Reasons for exception to octet rule: 

Many compounds follow octet rule in which each atom is surrounded by 8 electrons (except for hydrogen surrounded by two electrons). Example, NH₃; nitrogen-central atom- have 5 electrons so it surrounds itself with 3 hydrogen atoms to satisfy octet rule. For that, nitrogen in NH₃ compound has 3 bonding pairs and one lone pair.

Yet, there are two categories of atoms that fail to satisfy octet rule. The first exceptions the atoms that have a fewer number of electrons than eight electrons. Example, NO has an odd number of electrons that cannot satisfy octet rule. 

The second category that fails to satisfy octet rule is divided into two groups; atoms that have valence electrons in molecule less than 8 and other group have electrons more than eight electrons. Example for central atoms that have a number of electrons less than 8 electrons is Boron in BCl₃ or BCl₂. Boron dichloride has four electrons around the atom and in boron trichloride, the boron has six electrons. 

The octet rule is easy to be satisfied for elements found in the first and second periods where ns and np valence shell orbitals are the bonding orbitals (2 electrons in s and 6 electrons in p = 8 electrons – octet rule). But moving down from period 3 on, the elements have unfilled nd orbitals, these orbitals make it hard to apply the octet rule (10 electrons). This happens with the second group (the atoms that can have electrons more than 8 electrons). Example, P can form both phosphorus trifluoride (PF₃) and phosphorus pentafluoride PF₅.

For phosphorus trifluoride, phosphorus [3s²3s²3p³] accepts three additional electrons in 3p orbital forming three covalent bonds (PF₃). Still, phosphorus can use the unboned lone pair to bond to another two fluorine atoms, by that phosphorus will accept these two electrons in 3d orbital forming phosphorus pentafluoride.