7. Electronic configuration of the Element..

According to Rutherford model of an atom, atom is made of outer electron cloud   (-vely charged electron) and inner nucleus (neutral neutron and +vely charged proton).Niels Bohr agreed with this concept , he also agreed that electron revolve round the nucleus like a planet moves round sun.He applied the Plank's quantum theory to the electron revolving round the nucleus
         The orbits , there fore called as energy levels or energy shells. Bohr gave numbers 1, 2, 3, ...etc to these energy levels, there are they now called as Principal quantum numbers.these energy levels are also designated by  k, l, m, n, etc...

The energy levels which are more far from the nucleus associated with greater amount of energy                                                        -The energy of outermost electron is greater than innermost.

-These outermost electron helps atom to react with other because they can pullout easily  from the orbital .

-Each shell made of many sub shells which are themselves composed of atomic orbitals.

-First shell K  have 1 sub shell = s

-Second shell L  = 2 sub shells = s , p

-Third shell M = 3 sub shells =s , p, d

-fourth shell N = 4 sub shells = s , p, ,d, f

No.	Sub shell	Name
1	s	Sharp
2	p	Principal
3	D	Diffused
4	f	fundamental

Shells	Sub shells	No. of electron	Type of sub shells found in shell	Distribution electron in sub shells
K	s	2	only s	1s
L	P	6	s ,p,	2s , 2p,
M	d	10	s, p, d,	3s , 3p, ,3d
N	f	14	s, p, ,d, f , d,	4s , 4p , 4d , 4f
…	g and so on	18…	s , p, d, f, g,…	Alphabetically  so on

-The reactivity of the element is highly dependent upon its electronic configuration.

- The distribution of electron in various orbital (s, p,d ,f ) is known as electronic configuration.

-Mathematically it is described by Slater rules.

-In representing position of electron in various shells and sub shells ,the following rules are observed..

=> Major  energy shells written first then sub shells and after that the no. of electron in particular sub shell

for example :-     

1s²,   where ,   1 represents the shell i.e. K

                        s represent the sub shell
                    and  2 indicates the no of electron present in sub shell.

-According to Afbau principal--

               The electrons enter in the various orbitals in the order of increasing energy.

- According to pauli exclusion principal--

                An orbital can contain a maximum of 2 electron and these two electrons must be in opposite spin.                                              

                                                                       

↑

↓

                                                            

-According to Hund's rule--

           It states that electron pairing in orbital of the same energy level will not take place unless all available orbitals of subshell contain one electron  each with parallel spin.   

Order of filling of electron in various orbitals is given below :--

Above picture shows normal periodic table of element, but from the picture given below we can find the outermost orbital of the element and electronic configuration.

There are some special cases of electronic configuration.
From periodic table we can see that the electronic configuration of the elements  are going in normal way till Argon(18) last electron goes in 3p orbital. But in case of Potassium(19) last electron should fill in 3d orbital but it is not so, it goes in 4s orbital instead of 3d orbital because 4s has low energy in comparison to 3d .Thus electron prefer to go in lower energy orbital then higher energy level.

-The next ten elements from ( Z=21 to 30) Sc to Zn are called Transition elements.
In these elements addition of electron take place in inner 3d orbitals while outer 4s orbital remains fully occupied.