state Aufbau , pauli 's excludion principle and hund's rule of maximum ..
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Answers
in the ground state of the atoms the orbitals are fikked with
electrons in their incresing order of energies. this is called aufbau principle.
hund's rule:
in the order of sub shell paairing of electrons takes place only after filling up of available orbitals with one elctron each.
the pauli's exclusion principle:
in a given atom no two electrons have the same set of quantum number.
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Aufbau principle
The Aufbau Principle is basically a German word “Aufbauen” which means building up.
According to this principle, the electrons will first occupy the orbitals that have the lowest energy. This means that the electrons enter first orbital and then enter the orbitals which have higher energy but only when the lower- energy orbitals are completely filled.
The order in which the electrons should be filled is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p…..
The order of the increasing energy of the orbitals can also be calculated using (n+l) rule.
The sum of the values of the principal quantum numbers (n) and azimuthal quantum number (l) is used to decide the energy level of an orbital.
The lower value of the sum of (n+l) implies that the energy of the orbital is low. If the value of n+l for two orbitals is equal then the orbital with a lower value of n will have a lower energy level.
Aufbau Principle
Pauli exclusion principle
According to this principle, an orbital can have maximum two electrons and these must have opposite spins.
In other words, no two electrons in an atom can have all the four quantum numbers exactly the same.
As we know that fourth quantum number is m, 1/2 is spin up and -1/2 is spin down and hence if the first three quantum numbers are same for two electrons then the electrons must have opposite spins in an orbital.
Hund’s rule
According to this rule, the electrons are filled in the degenerate orbitals of the same subshell.
Electron pairing in p, d and f orbitals cannot be done until each sub-shell is occupied singly.
This is because electrons are the same in charge and they repel each other. This repulsion is minimized if two electrons move away from each other by acquiring different degenerate orbitals.
All the orbitals which are singly occupied have parallel spins which can be either clockwise or anticlockwise.
If the electrons have parallel spin then there will be less inter-electronic repulsions and high spin multiplicity.
So in order to maximize spin multiplicity, the pairing of electrons in a sub-shell does not take place until each sub-shell is singly occupied.
Hund’s rule
Filling Electron Diagram by Hund’s Rule
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