Rules for Writing
Electronic Configuration
The atom is
built up by filling electrons in various orbital according to the certain rules
like Aufbau principle, Pauli Exclusion Principle, Hund’s rules
of maximum spin multiplicity etc. These are discussed below.
Aufbau Principle:-
This
principle state that the electrons are added one by one the various orbitals in
order of their increasing energy starting with the orbital of lowest
energy. The increasing order of various
atomic orbital is
1s, 2s, 2p,
3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 5f, 6d, 7p………………
How to
remember such a big sequence is a big deal because it appears to be highly
complicated. To make it simple, there is a method to write the increasing order
of the orbitals. Starting from the top, the direction of the arrows gives the
order of filling of orbital as shown below,
The sequence
of atomic orbital orbital shown above is based on (n+1) rule.
(n+1) Rule:-
The energy
of an orbital depends upon the sum of value of the principle quantum number (n)
and the azimuthal quantum number (l). This is called (n+1) rule. According to
rule, “ In neutral isolated atom, the
lower the value of (n+1) for an orbital, lower will be its energy However, if
the two different type of orbital have same value of (n+1), the orbital lower
value of n has lower energy”.
Pauli’s Exclusion Principle:-
According to
this principle “no two electron in an
atom will have same value of all four quantum number”.
If one electron
in an atom has the quantum number n=1, l=0, m=0 and s= +1/2, no other electrons
can have the same four quantum numbers. In other word , one cannot place two electron
with the same value of spin quantum number (s) in a 1s orbital. So, the orbital
diagram ##(1s) is not possible arrangement of
electrons. Rather, the correnct representation must be #$(1s).
Hund’s Rule of Maximum spin
multiplicity:-
This rule
deal with the filling of electron in the orbital having equal energy also known
as degenerate orbital’s. According to this rule “Degenerate orbital must be single occupied before the spin pairing
stars”.
This due to
the fact that electrons being identical in charge, repel each other when
present in the same orbital. This repulsion can however be two electrons move
as far as by possible by occupying different degenerate orbital’s. Moreover all
the unpaired electrons in a degenerate set of orbital’s must have the same
spin.
Electronic Configuration of
Various Elements:-
Electronic
configuration is the distribution of electrons into orbitals or shells,
subshell orbitals an atom. Keeping in view the above mention rule, electronic
configuration of any orbital can be simply represented by the notation. n l x
Where- n=
Principal quantum number
l= Symbol of subshell or orbital
(s, p, d, f)
x= Number of electrons in the
subshell
To write the
electronic configuration, one must know (1) The atomic number (2) The order
which orbital are to be filled (3) Maximum number of electrons that can
occupies in a shell, sub-shell or orbital. Some more fundamental aspects are
1. Each orbital can accommodate two
electrons.
2. The number of electrons to be
accommodated in a subshell is 2 × number of degenerate orbitals.
3. The maximum number of electrons in
each shell ( K, L, M, N…) is given by 2n2. When, n is the principal quantum
number.
Factors Responsible for the
Extra Stability of Half-Filled and completely filled Subshell
1. Symmetrical distribution:-
It is well
known fact that symmetry leads to stability. Thus the electronic configuration
in which all the orbitals of the same subshell are either completely filled or
are exactly half filled are more stable because of Symmetrical Distribution of electrons.
2.Exchange energy:-
The
electrons with parallel spins present in the degenerate orbital tend to exchange
their position. The energy released during this exchange is called Exchange Energy. The number of
exchanges that can take place is maximum when the degenerate orbital are
exactly half-filled or completely filled. As a result, the exchange is maximum
and is the stability.
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