SUBJECT :- CHEMISTRY
chapter :- 2
TOPIC:- Aufbau rule or Principle ,Hund's rule of maximum multiplicity and Pauli Exclusion Principle
Aufbau rule or Principle
The aufbau(German words its means building-up ) principle, from the German Aufbau principle (building-up principle), also called the aufbau rule The building up of orbitals means the filling up of electrons in orbitals
states that in the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels.
It states that in ground state of atom, the orbitals are filled in increasing order of their energies levels .as per Pauli Exclusion principle & Hund Rule
But Following ordering can be used to predict the configuration of electrons in atom
1s, 2s ,2p, 3s ,3p ,4s ,3d ,4p, 5s,4d,5p,6s,4f,5d,6p , 7s, 5f , 6d ,7p
The above order is based on Madelung Rule or BohrBurys rule ( n+l) rule which states that
(1).The sum of the values of the principal quantum numbers (n) and azimuthal quantum number (l) i.e (n+l) determined the energy level of an orbital.
Subshell having lower (n+)l value will be filled first
Example 3 s orbital (n+l = 3+0 = 3) will be filled before 4S (n+l = 4+0 = 4) orbital
(2). If the value of n+l for two orbitals is equal then the orbital with a lower value of principal quantum numbers (n) will have a lower energy level will be filled first
Example :- 2P orbital (n+l = 2 +1 = 3) and 3s orbital (n+l = 3+0= 3) have the same (n+l) value but 2p - orbital has lowest n (2) value & therefor electron filled first 2p subshell
Hund's rule of maximum multiplicity
.
Let's take an example of Carbon (C=6). If we follow the Aufbau rule and Pauli exclusion rule, we will have two electrons in the 2p subshell. Now arrange of those two electron can be any of these
Hund's rule identifies the first and last option as the electronic ground state.
This rule deals with the filling of electrons into the orbitals belonging to the same subshell
All the orbitals which are singly occupied have parallel spins which can be either clockwise or anticlockwise
Electrons have parallel spin, have less interelectronic repulsions and high spin multiplicity. The reason is that Electrons are negatively charged and, as a result, they repel each other. Electrons tend to minimize repulsion by occupying their own orbitals, rather than sharing an orbital with another electron.
Now we have three orbitals in p subshell , so pairing or doubling of electrons will start from 4th electron
Filling of electron in 2p6 subshell
Filling of electron from 2p1 to 2p6
P1 electronic configuration
P2 electronic configuration
P3 electronic configuration (stable electronic configuration of P3 )
P4 electronic configuration
P5 electronic configuration
P6 electronic configuration (stable electronic configuration of P6 )
An atom consists of a nucleus around which electrons revolve in orbitals of different energy. According to the Aufbau principle, electrons fill the lowest energy level before filling up the higher ones. Thus, electrons are found in discrete atomic orbitals in an arrangement known as electron configuration. However, filling up of orbitals follows a particular set of guidelines known as Hund’s rule, which states that:
1 The sublevel of every orbital is singly occupied before it is doubly occupied.
2 All electrons in singly occupied sublevel have the same spin in order to maximize the spin
Why Hund’s Rule is called the Rule of Maximum Multiplicity
According to Hund’s rule, the lowest energy term in a given electronic configuration has the highest value of spin multiplicity. The electrons enter the sublevel orbitals so that the maximum number of unpaired electrons occupy them. All of them have identical directions of spins. This electron configuration is known as maximum multiplicity.
Pauli Exclusion Principle
This principle was given by the Austrian scientist Wolfgang Pauli (1926)
The Pauli Exclusion Principle states that, in an atom or molecule, no two electrons can have the same four electronic quantum numbers. As an orbital can contain a maximum of only two electrons, the two electrons must three quantum number similar than they have opposing spins (spin quantum number)
The number of electrons to be filled in various orbitals is restricted by the exclusion principle
According to this principle no two electrons can have same set of 4 quantum number
For example, if two electrons reside in the same orbital, then their n, l, and m values are the same, therefore their s must be different, and thus the electrons must have opposite half-integer spin projections of ½ ↑ and -½ ↓.
It can be also stated as only two electron can exists in the orbital and these electrons must have opposite spin
EXAMPLE
for n=1
1s subshell can have maximum 2 electrons
Total Number of electron= 2
for n=2,
2s subshell can have maximum 2 electrons
2p -subshell can have maximum 6 electrons
Total Number of electron= 8
for n=3
3s subshell can have maximum 2 electrons
3p -subshell can have maximum 6 electrons
3d- subshell can have maximum 10 electrons
Total Number of electron= 18
This can be summed up as : the maximum number of electrons in the shell with principal quantum number n is equal to 2n2
. For the two electron in the orbital, the first three quantum number(n,l,
m) are the same, only the spin quantum number (s) differ. One electron will have up-↑spin ( +1/2), the other must be down↓-spin (-1/2)