Electronic configuration of atoms:
The distribution of electrons into different levels,sub-shells and orbitals of an atom is called its electronic configuration.The filling of orbitals is governed by the following principal.
1.Aufbau principle: (german:aufbau means' building up').
This principle states that:
"In the ground state of the atoms,the orbitals are filled in order of their increasing energies".
The increasing order of energies of the various orbitals is:
1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s........ .
The order of increase of energy of orbitals can be calculated from (n+i)rule or bohr's bury rule.
According to this rule, lower the value of n+i,lower is the energy of the orbital and such an orbital will be filled up first. For example,
(a) 4s orbital having lower value of (n+i)has lower energy than 3d orbital and hence 4s orbital is filled up first.
for 4s orbital,n+I=4+0=4
for 3d orbital,n+I=3+2=5
(b) When two orbital have same value of (n+i),the orbital having lower value of n has lower energy and hence is filled up first.
for example,2p orbital has lower value of n (=2)than 3s orbital(for which n is 3 ) and hence 2p will be filled up first.
for 2p orbital,n+I=2+1=3
for 3s orbital,n+I=3+0=3
2.pauli's exclusion principle:The principal was discovered by wolfgang pauli in 1925. According to it,no two electrons in an atom can have all the four quantum number same.This means that two electrons present in an atom can have a maximum of three quantum number same i.e.,they can be present in the same shell,same sub-shell and same orbital.However,they must have opposite spins i.email. , the spin quantum number are different.
In order to illustrate the principal,let us consider two electrons present in the k-shell.For them:
n=1 i=0 m=0 s=+1/2 and-1/2
(same) (same) (same) (same)
Thus it is clear that an orbital can have a maximum of two electrons with opposite spins.
Importance of the principle:It helps to find the number of electrons present in orbital, sub-shell and principal energy level.
For example:
when n=1,I=0,m=0,s=+-1/2
so there two electrons in first energy level.
when n=2,
(1) l=0,m=0,s=+-1/2
(2) I=1,m=-1,0,+1,s=+-1/2 for each value of m.
so there are eight electrons in the second energy level.
3.Hund's rule of maximum multiplicity:This rule states that :the pairing of electrons belonging to the same sub-shell (p,d or f) does not take place until each orbital belonging to that sub-shell has got one electron each i.e.,is singly filled."
For example,there are three p,five d and seven f orbitals,therefore,the pairing of electrons starts with the addition of 4th,6th and 8th electron in p,d and f sub-shell respectively.This is due to the fact electrons tend to remain farthest away from each other in order to have a state of minimum energy.
Representation of electronic configuration of atoms:The distribution of electrons into different shells,subshells and orbitals of an atom is called its electronic configuration. On the basis of the above rules, usually the electronic configuration of the atom of any element is simply represented by the notation.e.g.1s2 electrons are present in the s subshells of the 1st main shell.