chemistry electronc configuration

1. हर हुस्न लाजवाब नह ीं होता,
और हर क्यों का,
क्यों जवाब नह ीं होता।
AN INVESTIGATION
Madelung

2. The aufbau principle states: In the ground state
of an atom, atomic orbitals are filled by
electrons in the order of their increasing
energies. In other words, electrons will occupy
the lowest-energy orbital first before filling
higher-energy orbitals.

3. The etymology of aufbau
Reader may think aufbau is the name of a scientist, but it
is not. The word aufbau (auf-: up and bau: building) is a
german word meaning building up or construction.

4. Madelung rule. (n + l rule)
• from where did this rule originate. Is there
any scientific reason behind the principle?
Well, there is no such equation that dictates
this rule. But there is a scientific logic behind
it, and it is n + l rule.

5. • The energy of an orbital mainly depends on two
quantum numbers: the principal quantum number (n)
and azimuthal quantum number (l). The principal
quantum number corresponds to the shell while the
azimuthal quantum number, subshell. As the value of
both quantum numbers increases, the energy of an
orbital also increases.

6. The Madelung rule, named after German physicist
Erwin Madelung, states:
• Electrons in an atom fill the orbital with the
lowest value of n + l.
• When two or more orbitals have the same
value of n + l, the electron will occupy the
orbital with the lowest value of n.

7. • The energy of the orbital increases with n + l.
Whenever there is a tie in the value, the energy
increases with n.
• In the diagram, each orbital along the diagonal has
the same value of n + l (see below).

9. Many elements do not always follow the ordering of
orbitals as predicted by the aufbau principle.
• This is observed in transition metals,
lanthanides and actinides.

10. • Consider an example of scandium (Z = 21). Its electronic
configuration is [Ar] 4s2 3d1 as per the principle, but this
contradicts the spectroscopic observation. When the
scandium atom is ionized, we presume the electron will be
released from the highest energy orbital—3d orbitals as
stated by the principle. However, the electron is released
from 4s to form Sc+ having the electronic configuration
[Ar] 3d1 4s1. This suggests that 3d have more energy than
4s. This behaviour is also seen in the remaining transition
metals.

11. A similar trend exists in lanthanides and actinides. For example, the
aufbau filling order of neodymium (Z = 60) is [Xe] 6s2 4f4, but the
order of leaving is [Xe] 4f4 6s2.
For heavier nuclei (Z ≥ 120), the aufbau principle becomes invalid.
As the nuclear charge increases, the electrons, particularly nearer
to the nucleus, experiences a heavy electrostatic force. The
electrons of such nuclei have velocities approaching the speed of
light. Thus, we need to account for the relativity theory to the
quantum mechanic model.

12. • We have already mentioned the exceptions to the rule. But
why do we have exceptions? Is there any better rule than
this? The answer is no. There is no simple mathematical
formula that describes the electronic system. In an atom, we
have two electrostatic interactions: the attraction between
the positive nucleus and negative electrons, and the repulsion
among the negative electrons. Every atom or ion tries to
minimize the repulsion and try to reach the lowest energy
configuration. And understanding such a system is very
complicated, not easy.

13. Element Symbol
Atomic
number
Aufbau's prediction Experimental observed
Chromium Cr 24 [Ar] 4s2 3d4 [Ar] 4s1 3d5
Copper Cu 29 [Ar] 4s2 3d9 [Ar] 4s1 3d10
Niobium Nb 41 [Kr] 5s2 4d3 [Kr] 5s1 4d4
Molybdenum Mo 42 [Kr] 5s2 4d4 [Kr] 5s1 4d5
Ruthenium Ru 44 [Kr] 5s2 4d6 [Kr] 5s1 4d7
Rhodium Rh 45 [Kr] 5s2 4d7 [Kr] 5s1 4d8
Palladium Pd 46 [Kr] 5s2 4d8 [Kr] 4d10
Silver Ag 47 [Kr] 5s2 4d9 [Kr] 5s1 4d10
Platinum Pt 78 [Xe] 6s2 4f14 5d8 [Xe] 6s1 4f14 5d9
Gold Au 79 [Xe] 6s2 4f14 5d9 [Xe] 6s1 4f14 5d10
In each element, the d orbital takes an extra electron from the s orbital, except in palladium
where both electrons are consumed by the d orbital.

14. Element
Sym
bol
Atomi
c
numb
er
Aufbau's prediction Experimentally observed
Lanthanum La 57 [Xe] 6s2 4f1 [Xe] 6s2 5d1
Cerium Ce 58 [Xe] 6s2 4f2 [Xe] 6s2 4f1 5d1
Gadolinium Gd 64 [Xe] 6s2 4f8 [Xe] 6s2 4f7 5d1
Actinium Ac 89 [Rn] 7s2 5f1 [Rn] 7s2 6d1
Thorium Th 90 [Rn] 7s2 5f2 [Rn] 7s2 6d2
Protactinium Pa 91 [Rn] 7s2 5f3 [Rn] 7s2 5f2 6d1
Uranium U 92 [Rn] 7s2 5f4 [Rn] 7s2 5f3 6d1
Neptunium Np 93 [Rn] 7s2 5f5 [Rn] 7s2 5f4 6d1
Curium Cm 96 [Rn] 7s2 5f8 [Rn] 7s2 5f7 6d1
Lawrencium Lr 103 [Rn] 7s2 5f14 6d1 [Rn] 7s2 5f14 7p1

15. In all the above exceptions, the d orbital takes an electron from
the f orbital; thorium and lawrencium are special cases. In
thorium, 6d consumes both electrons from 5f while in
lawrencium 6d is replaced by 7p.
हर हुस्न लाजवाब नह ीं होता
और हर क्यों का जवाब नह ीं होता।