Here in this tutorial we will discuss about the cases where octet rule does not work.In many molecules atoms have electronic structure that stisfy the octet rule but a number of them do not.
An octet of electrons does not guarantee that an inner atom is in its most stable configuration.In perticular,elements that occupy the third and higher rows of the periodic table and have more than four valence electrons may be most stable with more than an octet of electrons.Atoms of these elements have valence d orbitals,which allow them to accomodate more than eight electrons.
Exceptions to the octet Rule: Electron deficient molecules:
There are molecules which do not obey octet rule,amoung these are the molecules containing an odd number of valence electrons like, NO and NO2 have 11 and 17 valenece electrons respectively.
For such odd electron species it is impossible to write Lewis structures in which each atom obeys the octet rule.There are few species in which central atom violates the octer rule in the sense that it is surrounded by two or three electrons pair rather than four,this belongs to the groups consists of mostly molecules containing group IIA or IIIA atoms.Like BF3, the B in the molecule is surrounded by much more electronegative atom F .The total number of valence electrons are 24 when you find the B has only six valence electrons.In chemistry of BF3 an electron structure with boron having only six electrons best supported.
Other examples of molecules with group III A atoms such as Al or group II A atoms such as Be display electron structures similar to that of Boron triflouride.
Exceptions to the octet Rule: Expanded Octets: This is the largets class of the molecules to violate the octet rule consists of species in which the central atom is surrounded by more than four pairs of valence electrons.Typical examples belongs to the atoms from third row phosphorus with five valence electrons can form as many as five bonds.Similary, sulfur,with six valence electrons,can form six bonds and chlorine with seven electrons can form as many as seven bond.
The octet rule stems from the fact that main-group elements in most cases employ only ns and three np valence shell orbitals in bonding and these orbitals holds eight electrons.Elements of the second period are restricted to these orbitals,but from the third period on the elements also have unfilled nd orbitals,which may be used in bonding.
Molecules like NO,PF5,XeF4 are simple example.
"when you try to draw the lewis structure it becomes clear that an expanded octet is involved.The number of electrons available after each skeleton is greater than the number of electrons required to give each atom an octet.When that happens,distribute the extra electrons around the central atom as unshared pairs."