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Electronic Configuration
Describe show the electrons of an atom are arranged in the energy levels, sub
shells and orbitals
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Quantum number
These numbers serve as the address of the electrons in an atom and hence are also
known as identification numbers
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e
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There are four quantum numbers viz
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(i) Principal or Radial quantum number
This quantum number represents the number of shell or main energy level to which
the electron belongs round the nucleus
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This quantum
number can have integral values 1,2, 3, 4………
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which
are designated by the letters K, L,M,N……
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as follows (proposed by Bohr):
Value of n Designation
1K
2L
3M
4N
It can be concluded that the principal quantum number (n) gives an idea of:
(a) The shell or main energy level which the electron belongs to
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e
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(c) The energy associated with the electron
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According to Bohr-Berry scheme, the maximum number of electrons in nth shell =
2n2
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(ii) Subsidiary quantum number (l)
Within the principal quantum energy shell there are subshells which are denoted
as s, p, d and f
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It is denoted by the letter l and refers
to the subshell which the electron belongs to
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The
values of l depend on the value of n (the principal quantum number) and may have
all possible values from 0 to (n-1), i
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, l = 0,1,2,3………
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Thus, for a given
value of n, total number of l values is equal to n, e
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, when n = 4, l = 0,1,2,3 (total
4 values of l)
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Thus, s stands for sharp, p for principal, d for
diffuse and f for fundamental
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The lowest energy orbitals are always filled first
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Sometime, lower (n + l) rule is violated
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g
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In general, the subshells with lower n
values are filled first followed by those with higher n values (called lower n rule)
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g
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In the former case, their spins must be antiparallel (different direction):
↓↑

↑

1s2

2s1

Rule 3: Hund's rule: The rule states that the electrons fill the orbitals of an atom
in such a way that they first occupy singly orbitals of equivalent energies before
they start being paired up in them
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g
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The p-orbital
consists of three degenerate orbitals, called Px, Py and Pz
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The order of filling in the electrons in atoms is summarized in the diagram below
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p …
Name of the Element Symbol with Atomic Number Electronic Configuration
Hydrogen H (Z = 1) 1s1
Helium He (Z = 2) 1s2
Lithium Li (Z = 3) [He] 2s1
Beryllium Be (Z = 4) [He] 2s2
Boron B (Z = 5) [He] 2s2 2p1
Carbon C (Z = 6) [He] 2s2 2p2
Nitrogen N (Z = 7) [He] 2s2 2p3
Oxygen O (Z = 8) [He] 2s2 2p4
Fluorine F (Z = 9) [He] 2s2 2p5
Neon Ne (Z = 10) [He] 2s2 2P6
Sodium (Natrium) Na (Z = 11) [Ne] 3s1
Magnesium Mg (Z = 12) [Ne] 3s2
Aluminium Al (Z = 13) [Ne] 3s2 3p
Silicon Si (Z = 14) [Ne] 3s2 3p2
Phosphorus P (Z = 15) [Ne] 3s2 3p3
Sulphur S (Z =16) [Ne] 3s2 3p4
Chlorine Cl (Z = 17) [Ne] 3s2 3p5
Argon Ar (Z = 18) [Ne] 3s2 3p6

If we try to write down the electronic configuration of potassium (Kalium) (K,
Z=19) according to above trend, the last electron must go to the 3d subshell, i
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K19 = [Ne] 3s2 3p6 3d1 or [Ar] 3d1 but this electron is said to enter the 4s–subshell
according to lower(n+l) rule of Aufbau principle
Sc (Z = 21) [Ar] 4s2 3d1
Ti (Z = 22) [Ar] 4s2 3d2
V (Z = 23) [Ar] 4s2 3d3
Cr (Z = 24) [Ar] 4s1 3d5
Mn (Z = 25) [Ar] 4s2 3d5
Fe (Z = 26) [Ar] 4s2 3d6
Co (Z = 27) [Ar] 4s2 3d7
Ni (Z = 28) [Ar] 4s2 3d8
Cu (Z = 29) [Ar] 4s2 3d10
Zn (Z = 30) [Ar] 4s23d10
The next six elements, that is, Ga31, to Kr36 belong to p-block and the last
electron enters the 4p – subshell of the atoms of these elements
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(s-block):
Rubedium Rb (Z = 37) [Kr] 5s1
Strontium Sr (Z = 38) [Kr] 5s2

Filling of 4d – subshell begins in the next element, yttrium, Y (Z = 39) and ends at
cadmium, Cd (Z = 48) {Y: [Kr] 5s2 4d2 to Cd: [Kr] 5s2 4d10}



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