Page 1 - LN 9

SUBJECT-CHEMISTRY

CHAPTER-ATOMIC STRUCTURE

SUBTOPIC- Pauli's exclusion Principle, Aufbau's Principle, Hund's Rule , node
and electronic configuration.

KEY NOTES :

 Pauli's exclusion Principle
 No two electrons in an atom can have the same set of four
quantum numbers.

 Only two electrons may exist in the same orbital and these
electrons must have opposite spin.
 This means that the two electrons can have the same value of
three quantum numbers n, l and ml , but must have the opposite
spin quantum number.
 The maximum number of electrons in the shell with principal
2
quantum number nis equal to 2n .
 Aufbau's Principle
 In the ground state of the atoms the orbitals are filled jn order of

their increasing energies.
 In other words, electrons first occupy the lowest energy orbital
available to them and enter into higher energy orbitals only after
the lower energy orbitals are filled.

 The order in which the energies of the orbitals increase and hence
the order in which the orbitals are filled is as follows :
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 4f, 5d, 6p, 7s...
 Arrangement of Orbitals with Increasing Energy on the Basis of
(n+l) Rule

 Hund's Rule
 Pairing of electrons in the orbitals belonging to the same
subshell (p, d or f) does not take place until each orbital
belonging to that subshell has got one electron each i.e., it is
singly occupied.

 Since there are three p, five d and seven f orbitals, therefore, the
pairing of electrons will start in the p, d and f orbitals with the
entry of 4th, 6th and 8th electron, respectively.