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Electronic Configuration

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Aufbau Rule:

Aufbau in german means building up or construction.It helps in the electronic configuration of elements (we will come to that later). It means that electrons in various energy levels in an atoms are filled in the increasing order of their energy.

The energy of any energy level is proportional to the value of (n + l ). So, the orbital with a smaller value of (n + l ) is filled first.If, two orbitals have same value of n+l , the orbital with a smaller value of n is filled first.

The order of energy is given by the order : 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 4f.

The above order of energy is used in the determining the block in which the element belongs to in the periodic table.

The block is decided by the block the last electron of the element goes to .The last electron is also called differentiating electron.

For example, the electronic configuration of the element of Fluorine (atomic number 9) is 1s2 2s2 2p5 .

 

 

 

Orbitals in the increasing order of energy (source: http://darkwing.uoregon.edu/)

 

 

 

Hundsinglequotes Rule of maximum multiplicity: 

Hundsinglequotes Rule in simplified terms states that before any two electrons occupy an orbital in a subshell, other orbitals in the same subshell must first each contain one electron, which means that before pairing happens in a px orbital , both py and pz orbital must contain one electron.

 

This gives stability to atoms, as electrons present in same orbital will repel each other more than electrons of different orbitlal.Moreover, all orbital containing one electron have parallel spins, and electron from different orbitals with parallel spins experience less inter electronic repulsion than electrons from different orbitals with opposite spins.The unpaired electrons (with parallel spins) provide a net higher spin than than the paired electrons.So, in case of unpaired electron the total spin is higher (or the multiplicity is maximum) .

 

Electronic Configuration:

 Electronic configuration is nothing but the arrangement of atoms in various shells, subshells and orbitals of an atom. There are two ways of representing the electronic configuration either by a box and showing the electron by their spins ( an arrow pointing upwards or downwards)  or by writing the name of orbitals and the number of electrons contrained in them.Like the electronic configuration of Neon in this method is written as:

                           Ne (Z =10) = 1s2 2s2 2p6    or                 

   
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                                                          1s        2s      2px   2py   2pz

Similarly, electronics configuration of other compounds are :

                          Ar (Z =18) = 1s2 2s2 2p6  3s2 3p6 


                     Kr (z = 36) = 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6

The electronic configuration is always written following the Aufbau Rule and Hundsinglequotes Rule, but there are a few exceptions like Cr (Z = 24) and Cu (Z = 29) . It is because the 3d and 4s orbitals do not have a large energy gap so an electron from 4s orbital jumps to 3d orbital to have a more stable or more symmetrical configuration.


Therefore, Cr(24) has the electronic configuration of [Ar] 4s1 3d5 instead of [Ar] 4s2 3d4.

And, Cu(29) has electronic configuration of [Ar] 4s1 3d10 instead of [Ar] 4s2 3d9.


The electronic configuration of Cu and Cr is more stable this way. There are two factors which explain this phenomenon :

a)Symmetrical arrangement: Half filled or completely filled orbitals are more stable because electrons are more symmetrically distributed.


b) Exchange energy: Electrons with parallel spins in degenerate orbitals often exchange their positions.Whenever the exchage their position, energy is released.This energy is called exchange energy.So, more energy will be released if there are more of such exchanges.These exchanges are maximum if the orbitals are exactly half filled or completely filled.Thats why completely filled or half filled orbitals are preferred.


 Based on the above theory , the electronic configuration of elements upto atomic number 86 is given below.


 
 
Atomic Number
Symbol
K
L
M
N
O
P
Q
1st  Period
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
5s
5p
5d
5f
6s
6p
6d
6f
7s
7p
1
1
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
2
2
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
2nd  Period
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
5s
5p
5d
5f
6s
6p
6d
6f
7s
7p
3
2
1
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
4
2
2
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
5
2
2
1
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
6
2
2
2
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
7
2
2
3
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
8
2
2
4
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
9
2
2
5
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
10
2
2
6
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
3rd  Period
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
5s
5p
5d
5f
6s
6p
6d
6f
7s
7p
11
2
2
6
1
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
12
2
2
6
2
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
13
2
2
6
2
1
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
14
2
2
6
2
2
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
15
2
2
6
2
3
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
16
2
2
6
2
4
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
17
2
2
6
2
5
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
18
2
2
6
2
6
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
4th Period
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
5s
5p
5d
5f
6s
6p
6d
6f
7s
 
19
2
2
6
2
6
..
1
 
 
 
 
 
 
 
 
 
 
 
 
 
20
2
2
6
2
6
..
2
 
 
 
 
 
 
 
 
 
 
 
 
 
21
2
2
6
2
6
1
2
 
 
 
 
 
 
 
 
 
 
 
 
 
22
2
2
6
2
6
2
2
 
 
 
 
 
 
 
 
 
 
 
 
 
23
2
2
6
2
6
3
2
 
 
 
 
 
 
 
 
 
 
 
 
 
24
2
2
6
2
6
5
1
 
 
 
 
 
 
 
 
 
 
 
 
 
25
2
2
6
2
6
5
2
 
 
 
 
 
 
 
 
 
 
 
 
 
26
2
2
6
2
6
6
2
 
 
 
 
 
 
 
 
 
 
 
 
 
27
2
2
6
2
6
7
2
 
 
 
 
 
 
 
 
 
 
 
 
 
28
2
2
6
2
6
8
2
 
 
 
 
 
 
 
 
 
 
 
 
 
29
2
2
6
2
6
10
1
 
 
 
 
 
 
 
 
 
 
 
 
 
30
2
2
6
2
6
10
2
 
 
 
 
 
 
 
 
 
 
 
 
 
31
2
2
6
2
6
10
2
1
 
 
 
 
 
 
 
 
 
 
 
 
32
2
2
6
2
6
10
2
2
 
 
 
 
 
 
 
 
 
 
 
 
33
2
2
6
2
6
10
2
3
 
 
 
 
 
 
 
 
 
 
 
 
34
2
2
6
2
6
10
2
4
 
 
 
 
 
 
 
 
 
 
 
 
35
2
2
6
2
6
10
2
5
 
 
 
 
 
 
 
 
 
 
 
 
36
2
2
6
2
6
10
2
6
 
 
 
 
 
 
 
 
 
 
 
 
5th Period
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
5s
5p
5d
5f
6s
6p
6d
6f
7s
7p
37
2
2
6
2
6
10
2
6
..
..
1
 
 
 
 
 
 
 
 
 
38
2
2
6
2
6
10
2
6
..
..
2
 
 
 
 
 
 
 
 
 
39
2
2
6
2
6
10
2
6
1
..
2
 
 
 
 
 
 
 
 
 
40
2
2
6
2
6
10
2
6
2
..
2
 
 
 
 
 
 
 
 
 
41
2
2
6
2
6
10
2
6
4
..
1
 
 
 
 
 
 
 
 
 
42
2
2
6
2
6
10
2
6
5
..
1
 
 
 
 
 
 
 
 
 
43
2
2
6
2
6
10
2
6
6
..
1
 
 
 
 
 
 
 
 
 
44
2
2
6
2
6
10
2
6
7
..
1
 
 
 
 
 
 
 
 
 
45
2
2
6
2
6
10
2
6
8
..
1
 
 
 
 
 
 
 
 
 
46
2
2
6
2
6
10
2
6
10
..
..
 
 
 
 
 
 
 
 
 
47
2
2
6
2
6
10
2
6
10
..
1
 
 
 
 
 
 
 
 
 
48
2
2
6
2
6
10
2
6
10
..
2
 
 
 
 
 
 
 
 
 
49
2
2
6
2
6
10
2
6
10
..
2
1
 
 
 
 
 
 
 
 
50
2
2
6
2
6
10
2
6
10
..
2
2
 
 
 
 
 
 
 
 
51
2
2
6
2
6
10
2
6
10
..
2
3
 
 
 
 
 
 
 
 
52
2
2
6
2
6
10
2
6
10
..
2
4
 
 
 
 
 
 
 
 
53
2
2
6
2
6
10
2
6
10
..
2
5
 
 
 
 
 
 
 
 
54
2
2
6
2
6
10
2
6
10
..
2
6
 
 
 
 
 
 
 
 
6th Period
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
5s
5p
5d
5f
6s
6p
6d
6f
7s
7p
55
2
2
6
2
6
10
2
6
10
..
2
6
..
..
1
 
 
 
 
 
56
2
2
6
2
6
10
2
6
10
..
2
6
..
..
2
 
 
 
 
 
57
2
2
6
2
6
10
2
6
10
..
2
6
1
..
2
 
 
 
 
 
58
2
2
6
2
6
10
2
6
10
2
2
6
..
..
2
 
 
 
 
 
59
2
2
6
2
6
10
2
6
10
3
2
6
..
..
2
 
 
 
 
 
60
2
2
6
2
6
10
2
6
10
4
2
6
..
..
2
 
 
 
 
 
61
2
2
6
2
6
10
2
6
10
5
2
6
..
..
2
 
 
 
 
 
62
2
2
6
2
6
10
2
6
10
6
2
6
..
..
2
 
 
 
 
 
63
2
2
6
2
6
10
2
6
10
7
2
6
..
..
2
 
 
 
 
 
64
2
2
6
2



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