What are Quantum numbers?
Electrons revolve around the nucleus at different energy levels. There are
four quantum numbers (n, l, m, and s) that describe the size, shape,
orientation, and spin directions of energy levels of electrons in an atom.
These numbers are called quantum numbers.
These are-
a) Principal Quantum Number(n)
b) Subsidiary Quantum Number(l)
c) Magnetic Quantum Number(m) and
d) Spin Quantum Number(s).
With the help of four quantum numbers, an electron can be described easily.
Knowing the address, a person can be easily identified. Similarly, knowing the
values of four quantum numbers, an electron can easily be identified. The four
quantum numbers are disscussed below:
a) Principal Quantum Number
According to Bohr, the electrons revolve around the nucleus at certain energy
levels. The numbers which designate the stationary energy levels are called
principal quantum numbers. The principal quantum number is expressed by the
letter, n. n has a whole number value, 1, 2, 3, ……… etc.
b) Subsidiary Quantum Number
Each electron revolving around principal energy levels is divided into some
sublevels. These sublevels indicate the shape of the electron’s movement. This
type of quantum number is used to locate the position of electrons in a
sublevel of a particular principal energy level. This quantum number is
denoted by the letter ‘l’. ‘l’ has values from 0 to n-1. The subsidiary
quantum number indicates the shape of the orbital.
For example, if n = 1, l = 0
if n = 2, l = 0, 1
if n = 3, l = 0, 1, 2
c) Magnetic Quantum Number
The earth has a magnetic field. The effects of positively charged proton and
negatively charged electrons in atoms tend to create a magnetic field. These
two magnetic fields always tend to affect the movement of electrons. As a
result, the electron’s motion is oriented by the magnetic field. Thus, the
magnetic quantum number is used to indicate the orientation of orbitals. This
is expressed by ‘m’.
The value of m depends on the subsidiary quantum number, l. ‘m’ values can
range from +l to -l with zero included. Mathematically, m = ±l.
d) Spin Quantum Number
The spin quantum number represents the direction of the electron spin and is
denoted by s. The direction of spin is investigated in the presence of
external magnetic field and it is found that all electrons spin either in one
direction (say, clockwise direction) or in the direction directly opposite
(anti-clockwise).
The spin quantum numbers can have values ±½ or ½ which are the mathematical
notations for the spin direction of electrons.
