Question
Describe Aufbau Principle.
Solution
Aufbau Principle (Aufbau means building up): According to this principle, the electrons are filled in various orbitals in order of their increasing energies. Thus, an orbital with the lowest energy will be filled first. The energy of an orbital is determined by (n + l) rule as explained below:
(i) The orbital with the lower (n + l) value has lower energy. For example, 4s and 3d orbitals.
Thus, 4s orbital has a lower energy than 3d orbital, hence filled first.
(ii) When two orbitals possess the same value of (n +1), then orbital having lower value of n has lower energy. For example, 2p and 3s orbitals.
(i) The orbital with the lower (n + l) value has lower energy. For example, 4s and 3d orbitals.
Thus, 4s orbital has a lower energy than 3d orbital, hence filled first.
(ii) When two orbitals possess the same value of (n +1), then orbital having lower value of n has lower energy. For example, 2p and 3s orbitals.
2p orbital 3s orbital
n = 2 n = 3
l = 1 l = 0
n + l = 3 n + l = 3
Thus, 2p orbital has a lower energy than 3s orbtial
n = 2 n = 3
l = 1 l = 0
n + l = 3 n + l = 3
Thus, 2p orbital has a lower energy than 3s orbtial
(iii) The different orbitals in the increasing order of energies are:
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s.
The order is diagrammatically represented as shown.
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s.
The order is diagrammatically represented as shown.
