Question

State the following:
a) Pauli's exclusion principle
b) Hund's rule of maximum multiplicity ​

Answer 1

a) Pauli's exclusion principle:
According to this principle, an orbital can have maximum two electrons and these must have opposite spins.
In other words, no two electrons in an atom can have all the four quantum numbers exactly the same.
As we know that fourth quantum number is m, +1/2 is spin up and -1/2 is spin down and hence if the first three quantum numbers are same for two electrons then the electrons must have opposite spins in an orbital.

b) Hund's rule of maximum multiplicity:
It states that, pairing of electrons in the orbital belonging to the same subshell does not take place until each orbital belonging to that subshell has got one electron each, i.e. it has to be singly occupied first, after that pairing of electrons takes place.