According to quantum
theory, it’s impossible to know the exact position and momentum of an electron
at the same time. The quantum mechanical model of the atom uses complex shapes
of orbitals, volumes of
space in which there is likely to
be an electron. So, this model is based on probability rather than certainty.
Four numbers,
called quantum numbers,
were introduced to describe the characteristics of electrons:
The Principal Quantum
Number (n)
The principal quantum
number n describes the average distance of the orbital from the nucleus — and
the energy of the electron in an atom. It can have positive integer values i.e.
n= 1, 2, 3, 4, and so on.
The Angular Momentum
Quantum Number (l)
The angular momentum
quantum number l describes
the shape of the orbital, and the shape is limited by the principal quantum
number n: The angular momentum quantum number l can have positive integer values from 0 to n–1. For
example, if the n value is 3, three values are allowed for l: 0, 1, and 2. Orbitals that have
the same value of n but different values of l are called subshells.
Letters
used for denoting of the subshells
|
|
Value
of l (subshell)
|
Letter
|
0
|
s
|
1
|
p
|
2
|
d
|
3
|
f
|
4
|
g
|
The Magnetic Quantum Number (ml)
The magnetic quantum
number describes how the various orbitals are oriented in space. The value of
this number depends on the value of l. The values allowed are
integers from –l to +l. For example, if the value of l =
1, you can write three values for ml
: –1, 0, and +1.
The Spin Quantum Number (ms)
The fourth and final
quantum number is the spin quantum number describes the direction the electron
is spinning in a magnetic field — either clockwise or anticlockwise. Only two
values are allowed: +1/2 or –1/2. For each subshell, there can be only two
electrons, one with a spin of +1/2 and another with a spin of –1/2.
Electronic
Configurations of Atoms
The
distribution of electrons in different orbitals of an atom is called the
electronic configuration of the atom. The filling of electrons in orbitals is
governed by the following rules.
1. Pauli's
exclusion principle: no
two electrons in an atom can have all the four quantum number (n, l, ml and ms)
the same.
2. Aufbau
principle: Electrons
enter the orbitals of lowest energy first.
3. Hund's
Rule: When electrons are
added to a subshell where more than one orbital of the same energy is
available, their spins remain parallel. They occupy different orbitals until
each one of them has at least one electron.
