Dr Inam Ul Haq Jazbi Electronic Configuration and Its Rules
Electronic Configuration and its Rules
Definition Electronic Configuration
The distribution of electrons in the available orbitals in the order of increasing energy
is called Electronic Configuration.
The electronic configuration describes the exact position of electron in extra-nuclear region i.e. available
orbitals. Thus electronic configuration is the filling up of orbitals in the sequence of increasing energy.
Some Unexpected Electronic Configuration
1. The elements of group VIB i.e. Cr, Mo and W have expected valence shell electronic configuration
(n-1)d4, ns2 but in practise their configuration is (n-1)d5, ns1.
e.g.
Chromium has 24 electrons and its expected electronic configuration is 1s2, 2s2 2p6, 3s2 3p6, 3d4, 4s2
but in reality the configuration is 1s2, 2s2 2p6, 3s2 3p6, 3d5, 4s1.
2. The elements of group IB i.e. Cu, Ag and Au have expected valence shell electronic configuration
(n-1)d9, ns2 but in practise their configuration is (n-1)d10, ns1.
e.g.
Copper with Z = 29 has the electronic configuration of 1s2, 2s2 2p6, 3s2 3p6, 3d10, 4s1 instead of
1s2, 2s2 2p6, 3s2 3p6, 3d9, 4s1
Factors responsible for the Extra Stability of Half-filled and Complete filled Sub-shells
(i) Symmetrical Electronic Distribution
The symmetrical electronic distribution leads to stability. Thus the electronic configuration with all the orbitals of
the same subshell are either fully filled or exactly half filled are more stable due to symmetrical distribution
of electrons.
(ii) Exchange energy
The electrons with parallel spins present in the degenerate orbitals (orbitals of same subshell having equal
energy) tend to exchange their position. The energy released during this exchange is called exchange energy.
The number exchanges that can take place is maximum when the degenerate orbitals are exactly half filled or
fully filled. As a result, the exchange energy is maximum and so it the stability.
Methods of Writing Electronic configuration
1. Orbital Method
In this method, the electrons present in respective orbitals are denoted.
Cl (17) = 1s2, 2s2 2p6, 3s2 3p5 OR [Ne] 3s2 3p5
2. Shell method
In this method, the number of electrons in each shell is continuously written
Cl (17) = 1s2, 2s2 2p6, 3s2 3p5
K2, L8, M7
2, 8, 7
3. Box Method
In this method, each orbital is denoted by a box and electrons are represented by half-headed (1) or full-
headed () arrows. An orbital can occupy a maximum of two electrons
4. Core Noble gas EC Method
In this method, the electrons present in the valence shells are shown while electrons in core shells are shown
by writing nearest noble gas in square bracket.
Cl (17) = 1s2, 2s2 2p6, 3s2 3p5 OR [Ne] 3s2 3p5
, Dr Inam Ul Haq Jazbi Electronic Configuration and Its Rules
Exceptions in Electronic Configurations
24Cr = [Ar] 3d5 4s1 (not 3d4 4s2)
29Cu = [Ar] 3d10 4s1 (not 3d9 4s2)
41Nb = [Kr] 4d4 5s1 (not 4d3 5s2)
42Mo = [Kr] 4d5 5s1 (not 4d4 5s2)
44Ru = [Kr] 4d7 5s1 (not 4d6 5s2)
45Rh = [Kr] 4d8 5s1 (not 4d7 5s2)
46Pd = [Kr] 4d10 5s0 (not 4d8 5s2)
47Ag = [Kr] 4d10 5s1 (not 4d9 5s2)
57La = [Xe] 4f0 5d1 6s2 (not 4f1 5d0 6s2)
58Ce = [Xe] 4f2 5d0 6s2 (not 4f1 5d1 6s2)
64Gd = [Xe] 4f7 5d1 6s2 (not 4f8 5d0 6s2)
78Pt = [Xe] 5d9 6s1 (not 5d8 6s2)
79Au = [Xe] 5d10 6s1 (not 5d9 6s2)
89Ac = [Rn] 5f0 6d1 7s1 (not 5f1 6d0 7s2)
90Th = [Rn] 5f0 6d2 7s2 (not 5f2 6d0 7s2)
91Pa = [Rn] 5f2 6d1 7s2 (not 5f3 6d0 7s2)
92U = [Rn] 5f3 6d17s2 (not 5f4 6d0 7s2)
93Np = [Rn] 5f4 6d17s2 (not 5f5 6d0 7s2)
96Cm = [Rn] 5f7 6d17s2 (not 5f8 6d0 7s2)
111Rg = [Rn] 6d10 7s1 (not 6d9 7s2)
Electronic Configuration and its Rules
Definition Electronic Configuration
The distribution of electrons in the available orbitals in the order of increasing energy
is called Electronic Configuration.
The electronic configuration describes the exact position of electron in extra-nuclear region i.e. available
orbitals. Thus electronic configuration is the filling up of orbitals in the sequence of increasing energy.
Some Unexpected Electronic Configuration
1. The elements of group VIB i.e. Cr, Mo and W have expected valence shell electronic configuration
(n-1)d4, ns2 but in practise their configuration is (n-1)d5, ns1.
e.g.
Chromium has 24 electrons and its expected electronic configuration is 1s2, 2s2 2p6, 3s2 3p6, 3d4, 4s2
but in reality the configuration is 1s2, 2s2 2p6, 3s2 3p6, 3d5, 4s1.
2. The elements of group IB i.e. Cu, Ag and Au have expected valence shell electronic configuration
(n-1)d9, ns2 but in practise their configuration is (n-1)d10, ns1.
e.g.
Copper with Z = 29 has the electronic configuration of 1s2, 2s2 2p6, 3s2 3p6, 3d10, 4s1 instead of
1s2, 2s2 2p6, 3s2 3p6, 3d9, 4s1
Factors responsible for the Extra Stability of Half-filled and Complete filled Sub-shells
(i) Symmetrical Electronic Distribution
The symmetrical electronic distribution leads to stability. Thus the electronic configuration with all the orbitals of
the same subshell are either fully filled or exactly half filled are more stable due to symmetrical distribution
of electrons.
(ii) Exchange energy
The electrons with parallel spins present in the degenerate orbitals (orbitals of same subshell having equal
energy) tend to exchange their position. The energy released during this exchange is called exchange energy.
The number exchanges that can take place is maximum when the degenerate orbitals are exactly half filled or
fully filled. As a result, the exchange energy is maximum and so it the stability.
Methods of Writing Electronic configuration
1. Orbital Method
In this method, the electrons present in respective orbitals are denoted.
Cl (17) = 1s2, 2s2 2p6, 3s2 3p5 OR [Ne] 3s2 3p5
2. Shell method
In this method, the number of electrons in each shell is continuously written
Cl (17) = 1s2, 2s2 2p6, 3s2 3p5
K2, L8, M7
2, 8, 7
3. Box Method
In this method, each orbital is denoted by a box and electrons are represented by half-headed (1) or full-
headed () arrows. An orbital can occupy a maximum of two electrons
4. Core Noble gas EC Method
In this method, the electrons present in the valence shells are shown while electrons in core shells are shown
by writing nearest noble gas in square bracket.
Cl (17) = 1s2, 2s2 2p6, 3s2 3p5 OR [Ne] 3s2 3p5
, Dr Inam Ul Haq Jazbi Electronic Configuration and Its Rules
Exceptions in Electronic Configurations
24Cr = [Ar] 3d5 4s1 (not 3d4 4s2)
29Cu = [Ar] 3d10 4s1 (not 3d9 4s2)
41Nb = [Kr] 4d4 5s1 (not 4d3 5s2)
42Mo = [Kr] 4d5 5s1 (not 4d4 5s2)
44Ru = [Kr] 4d7 5s1 (not 4d6 5s2)
45Rh = [Kr] 4d8 5s1 (not 4d7 5s2)
46Pd = [Kr] 4d10 5s0 (not 4d8 5s2)
47Ag = [Kr] 4d10 5s1 (not 4d9 5s2)
57La = [Xe] 4f0 5d1 6s2 (not 4f1 5d0 6s2)
58Ce = [Xe] 4f2 5d0 6s2 (not 4f1 5d1 6s2)
64Gd = [Xe] 4f7 5d1 6s2 (not 4f8 5d0 6s2)
78Pt = [Xe] 5d9 6s1 (not 5d8 6s2)
79Au = [Xe] 5d10 6s1 (not 5d9 6s2)
89Ac = [Rn] 5f0 6d1 7s1 (not 5f1 6d0 7s2)
90Th = [Rn] 5f0 6d2 7s2 (not 5f2 6d0 7s2)
91Pa = [Rn] 5f2 6d1 7s2 (not 5f3 6d0 7s2)
92U = [Rn] 5f3 6d17s2 (not 5f4 6d0 7s2)
93Np = [Rn] 5f4 6d17s2 (not 5f5 6d0 7s2)
96Cm = [Rn] 5f7 6d17s2 (not 5f8 6d0 7s2)
111Rg = [Rn] 6d10 7s1 (not 6d9 7s2)
